Shinnok in general is just kinda a mess. It's like before Scooby-Doo and The Mystery Machine show up to crack the case. You have no idea who the culprit(s) are and you have no leads.
Weird metaphor, but I would argue across all his variation he's underwhelming. People kept saying Impostor, totally the way to go! Yet people tend to forget what looks good on paper is not always executed well. He's a rushed character in a game that does not focus on zoning and I have seen characters, many at this point with some type of jump arc or divekick that do not get hit by Hellsparks so I think if anything he just sucks simply because in any other game he might honestly be a good character.
But with his low damage output, the fact you mostly have to outplay your opponent to win with him, and try to get all this chip and spend meter just to be plus on anything and then all of a sudden you have no breaker because none of his shit is safe without meter aside from 1,1 b2 and 2. Not very good, is it?
Boneshaper gives him a chance. Necro, if given something else, might also give him a chance, but Impostor is just all gimmicks that once people find out, they get wrecked by it. You can't just air teleport and njk for free, it comes with a price. I even said earlier I highly think his vortex is overrated because of the damage and few options he can really use to set it up. (b3 into combo, f4 into hellsparks, grab? Okay...)
I will say that he probably could use a bit more of a damage buff and then people would actually have to be afraid of even getting touched by Shinnok, and seeing how damn unsafe his wakeup option is and how shitty shoulder is to begin with, and has odd hitboxes and whack strings that I think him being able to do more damage would honestly be all he needs. I just don't really get what to do with this character because well, then you add in the damage buff on Impostor and that might be too much, who knows.
NRS will likely buff him once again since the trend is bottom tier gets focused on way more than the top, but since he's in the lower mid/mid at best, who knows.