I have played this matchup at least over 1000 times with my fellow clanmate,
EMP Dark, and have beaten quite a few solid Jokers in tourney.
Superman vs. Joker is 7-3. This is coming from one of the few Supermans that actually relies on pure zoning in 90% of my matchups (95% in the Joker matchup), Superman can zone Joker just fine.
When you're at fullscreen and do an air laser halfway or lower to the ground, Joker cannot punish it. When Superman traits up, he can chip Joker out disgustingly bad with this strategy. Sure Joker can try to close in on Superman to have room to punish an air laser, but Superman has an amazing backdash and can run away very well. (the backdash can somewhat be used an option select when Joker does a gunshot to avoid it while you're running away) When Joker's gun whiffs, Superman has all day to air laser punish it.
Spamming gun shot at Superman is a bad idea to begin with because once Superman's feet leave the ground and you do a gunshot, Superman will punish you on whiff with air laser (God help you if he's traited up and has meter, bye bye 18% life). To be fair, if Superman is around 60% in the air, Joker can gunshot punish on a read. But when he air lasers, he kind of moves his feet back which can actually avoid a gunshot. This is a double edged sword for Joker because the gunshot heavily pushes Superman back on a punish.
Now that Joker has to respect Superman's feet leaving the ground, this open up Superman to be able to throw out a ground laser/straight laser or two in the ground game.
Yes, it "has like 1000 frame start up and is easy to see coming yadayadayada" , but that's why we Superman players don't spam that just as much as Joker players don't spam ground gun. Its to keep both opponents in check.
As far as CQC game goes, Joker's jump back 3 is a very good option select to avoid incoming f23s. Superman can straight laser punish a jumpback 3 on a read though (which is also a good strategy against Deathstroke's jumpback 3).
Also, just because your opponent has a standing 1 to punish F23 doesn't necessarily mean that's a good thing. Almost every character has that. Superman players will be aware of this and will be baiting a whiff punish, since F23 is arguably the best normal whiff punisher in the game. (F23breathf23breath is like sooooo April 17th, 2013).
Superman players should have positional awareness, since all this zoning yadada said above will eventually push us into the corner. Lets just try to assume that corner in said stage has no options to get us out. If Superman reads that you're trying to jumping in after a teeth setup, he can rising grab. Obviously you can turtle around and bait this, but I've seen myself sometimes backdash out of a teeth setup everytime Dark tries to duck and bait a rising grab punish.
I don't know too much about Jokers corner game but I know its still something to be reckoned with, which is why this matchup isn't 8-2 or 9-1 or 10-0. But I will say this. We are Superman, we have the tools to run away, we have the tools to keep Joker out, we have the meterbuilding to keep MB lasers stocked up for nasty chip damage and making our mistimed air lasers make us come out safely. We have the trait to chip Joker out faster than you can say "I'm going to corner you in this match". We have the options to select stages that give us "Get off me" options in the corner. We have the F23 to bait whiffed normals and dash ins.
We shouldn't succumb to Joker's corner game or even let Joker knock us down in CQC to begin with.