After finally taking the time to actually learn Tremor, I've come to the conclusion that he really is just a better version of Jax. I usually hate saying that because it almost never really applies, but in this case he simply does what Jax does, but better. Let's look at their tools:
[Midscreen]
Jax's midscreen game relies on (recently nerfed) frame trap pressure along with his excellent f21 string and the threat of dash punch. The only way to get any real damage is to burn meter on Overhead Punch (full-combo punishable) or play Pumped Up and abuse Gotcha. Dash Punch is -7 but can be ducked for a full-combo punish. It also only deals 10% damage with 26 hit advantage and its wonky hitbox can lead to it whiffing in certain situations.
Tremor, on the other hand, has arguably better frame trap pressure in the form of f121 (which literally goes full screen and is +2 on block). His 14-frame mid-hitting b1 rivals the range of Jax's f2, but thanks to his move set, b1 leads into a solid overhead/low mixup game into full midscreen combos. His 21 string has about twice as much range and leads to the same mixups and combos as b1. He also has his amazing b2 which serves as both poke and launcher. His f4 is a +6 poke with enough Hit Advantage to do whatever he wants if it connects (including 12% unavoidable otg damage for 1 bar). Tremor's strings also lead to safe 30%+ midscreen damage into massive Hit Advantage. Jax lacks the safety, damage, and advantage of Tremor's midscreen game.
Tremor's Rock Punch is every Jax player's wet dream. It's slightly slower at 13 frames, but it's a -7 mid with 30 Hit Advantage into a hard knockdown. It has almost the range of Dash Punch, but it can't be crouched. It also deals a solid 12% or 16%(!) when EX'd. When canceled into either of the launcher versions, it's also 4 frames safer than Overhead Punch and it leads to meterless midscreen damage, something Overhead Punch isn't capable of. It's better than Dash Punch (and Overhead Punch) in every way except for speed.
For zoning, Jax's Energy Wave is a very solid tool, and Pumped Up has a strong zoning game between Energy Wave, Ground Pound, IA Air Pound, and EX Ground Pound. However, these tools don't compare to Tremor's arsenal of ranged specials, particularly his low-hitting projectile. Since it's only -5 on block, it can be thrown into his pressure as a solid mix-up. Jax gets +7 block advantage off Energy Wave; however, it can be crouched or low-poked, and there is too much pushback to really capitalize on. If it hits, he gets nothing but 10 Hit Advantage. Tremor's projectile can't be crouched, grants 41 hit advantage, can be made +9 with meter (which jails into his amazing f121 and d1 and allows for an easy throw), can combo meterlessly in the corner, and deals more damage and chip. In Lava, he can get almost 40% meterless midscreen or corner damage from his low projectile.
In addition to the low projectile that is simply more dominant than Energy Wave in every way, he's got his own high projectile for block/hit advantage (that combos meterlessly in the corner for 30%+) along with his (admittedly lackluster) Up Rock moves, which do give him several viable corner setups.
Depending on Tremor's variation, his zoning ranges from average to very good. When comparing Pumped Up to Metallic (or even Aftershock), it's almost sad to see the chasm that exists between their zoning abilities.
Tremor's Ground Pound starts up 3 frames faster, though it does deal less damage and doesn't lead to a combo (like Wrestler Jax's), so I do have to give this one to Jax.
So at least when it comes to Midscreen, Tremor does everything Jax can do, but markedly better. How about the corner?
[Corner]
Here, things get a little more even, but I still believe Tremor does essentially what Jax does, but better. Jax has to rely on unsafe 50/50s to get his major damage. His 50/50 game is strong, but his Overhead starter and Overhead Punch are both full-combo punishable. He doesn't really have any "dirt" or setups in the corner either; just a strong 50/50 game into heavy damage and some decent stagger pressure. It's about as straightforward as you can get, especially since they nerfed his stagger game.
Tremor, on the other had, has options upon options. Whereas with Jax you just have to block the initial overhead or low, with Tremor you have to be on the lookout during the entirety of his pressure since he can go for an overhead or low launcher at any point during his strings (and he can keep you locked down by spending meter to keep +). His +2 and +6 strings lead to meterless 25%+ combos, giving him excellent stagger pressure into respectable damage. He has a +2 overhead that deals 14% damage. His launching overhead (db2d) is safer than either of Jax's overhead options. His low launcher can be made +9 for a bar (and is only -5 otherwise). He can end combos with Up Rock for "unblockable" setups. Lava can end all combos at +12 with the opponent standing (like Pumped Up's Gotcha, but 9 frames better...). Crystalline can end all combos into db2 Armor. Aftershock has those crazy mine setups and +-on-block IA Ground Pounds. Basically, Tremor out 50/50's Jax in addition to having an arsenal of additional dirt and setups at his disposal. He might not do as much damage consistently, but he's going to do his damage more safely and more often.
[Conclusion]
I want to stress that this isn't just a "Tremor is better than Jax" complaint. I do think he's better; the problem is that he's better at everything that Jax does. He's got a Dash Punch, but better. He's got Energy Wave, but better. He's got f21, but better. He's got corner mixups, but better. In fact, the only 2 things I can think of that Tremor doesn't have are Pump Up's EX Gotcha and Wrestler's tick throws. Those are great tools, but they don't change the fact that Tremor's existence makes Jax a redundant character in MKX. Jax is literally just a poor man's Tremor.
[Midscreen]
Jax's midscreen game relies on (recently nerfed) frame trap pressure along with his excellent f21 string and the threat of dash punch. The only way to get any real damage is to burn meter on Overhead Punch (full-combo punishable) or play Pumped Up and abuse Gotcha. Dash Punch is -7 but can be ducked for a full-combo punish. It also only deals 10% damage with 26 hit advantage and its wonky hitbox can lead to it whiffing in certain situations.
Tremor, on the other hand, has arguably better frame trap pressure in the form of f121 (which literally goes full screen and is +2 on block). His 14-frame mid-hitting b1 rivals the range of Jax's f2, but thanks to his move set, b1 leads into a solid overhead/low mixup game into full midscreen combos. His 21 string has about twice as much range and leads to the same mixups and combos as b1. He also has his amazing b2 which serves as both poke and launcher. His f4 is a +6 poke with enough Hit Advantage to do whatever he wants if it connects (including 12% unavoidable otg damage for 1 bar). Tremor's strings also lead to safe 30%+ midscreen damage into massive Hit Advantage. Jax lacks the safety, damage, and advantage of Tremor's midscreen game.
Tremor's Rock Punch is every Jax player's wet dream. It's slightly slower at 13 frames, but it's a -7 mid with 30 Hit Advantage into a hard knockdown. It has almost the range of Dash Punch, but it can't be crouched. It also deals a solid 12% or 16%(!) when EX'd. When canceled into either of the launcher versions, it's also 4 frames safer than Overhead Punch and it leads to meterless midscreen damage, something Overhead Punch isn't capable of. It's better than Dash Punch (and Overhead Punch) in every way except for speed.
For zoning, Jax's Energy Wave is a very solid tool, and Pumped Up has a strong zoning game between Energy Wave, Ground Pound, IA Air Pound, and EX Ground Pound. However, these tools don't compare to Tremor's arsenal of ranged specials, particularly his low-hitting projectile. Since it's only -5 on block, it can be thrown into his pressure as a solid mix-up. Jax gets +7 block advantage off Energy Wave; however, it can be crouched or low-poked, and there is too much pushback to really capitalize on. If it hits, he gets nothing but 10 Hit Advantage. Tremor's projectile can't be crouched, grants 41 hit advantage, can be made +9 with meter (which jails into his amazing f121 and d1 and allows for an easy throw), can combo meterlessly in the corner, and deals more damage and chip. In Lava, he can get almost 40% meterless midscreen or corner damage from his low projectile.
In addition to the low projectile that is simply more dominant than Energy Wave in every way, he's got his own high projectile for block/hit advantage (that combos meterlessly in the corner for 30%+) along with his (admittedly lackluster) Up Rock moves, which do give him several viable corner setups.
Depending on Tremor's variation, his zoning ranges from average to very good. When comparing Pumped Up to Metallic (or even Aftershock), it's almost sad to see the chasm that exists between their zoning abilities.
Tremor's Ground Pound starts up 3 frames faster, though it does deal less damage and doesn't lead to a combo (like Wrestler Jax's), so I do have to give this one to Jax.
So at least when it comes to Midscreen, Tremor does everything Jax can do, but markedly better. How about the corner?
[Corner]
Here, things get a little more even, but I still believe Tremor does essentially what Jax does, but better. Jax has to rely on unsafe 50/50s to get his major damage. His 50/50 game is strong, but his Overhead starter and Overhead Punch are both full-combo punishable. He doesn't really have any "dirt" or setups in the corner either; just a strong 50/50 game into heavy damage and some decent stagger pressure. It's about as straightforward as you can get, especially since they nerfed his stagger game.
Tremor, on the other had, has options upon options. Whereas with Jax you just have to block the initial overhead or low, with Tremor you have to be on the lookout during the entirety of his pressure since he can go for an overhead or low launcher at any point during his strings (and he can keep you locked down by spending meter to keep +). His +2 and +6 strings lead to meterless 25%+ combos, giving him excellent stagger pressure into respectable damage. He has a +2 overhead that deals 14% damage. His launching overhead (db2d) is safer than either of Jax's overhead options. His low launcher can be made +9 for a bar (and is only -5 otherwise). He can end combos with Up Rock for "unblockable" setups. Lava can end all combos at +12 with the opponent standing (like Pumped Up's Gotcha, but 9 frames better...). Crystalline can end all combos into db2 Armor. Aftershock has those crazy mine setups and +-on-block IA Ground Pounds. Basically, Tremor out 50/50's Jax in addition to having an arsenal of additional dirt and setups at his disposal. He might not do as much damage consistently, but he's going to do his damage more safely and more often.
[Conclusion]
I want to stress that this isn't just a "Tremor is better than Jax" complaint. I do think he's better; the problem is that he's better at everything that Jax does. He's got a Dash Punch, but better. He's got Energy Wave, but better. He's got f21, but better. He's got corner mixups, but better. In fact, the only 2 things I can think of that Tremor doesn't have are Pump Up's EX Gotcha and Wrestler's tick throws. Those are great tools, but they don't change the fact that Tremor's existence makes Jax a redundant character in MKX. Jax is literally just a poor man's Tremor.