Nothing. The entire concept is flawed from the start.
Let's imagine I'm making my own character: God the Hedgehog. He can do everything. Walk on water, invincible at all times, jump higher than a red spring and fly, you name it. What's interesting about this character when actually playing as him? Nothing. There's no vulnerabilities. There's nothing the character can't do. He has nothing to fear by any part of the game, and this makes him incredibly boring. What makes a character interesting is their weaknesses, not their strengths. When you play as Sonic in SRB2, he can't go up without outside assistance. He has to use springs, bounce, and do other things to gain vertical height. This vulnerability is core to our game's design, and all three characters have something they're bad at. SMS has none.
SMS has this problem at the very core of the design. He's simply a self-insert fanfic character, nothing more. There's nothing to fix here because the core is so rotten that it cannot be fixed. On top of this, SMS's author continuously bragged about his character like he was the best thing on God's green Earth. He bragged about (false) system requirements, and acted like SMS was SRB2's best feature. Hell, he
STILL does this at last check. I remember him porting that garbage to Smash and official Sonic games, and actually labeling him as "from SRB2" like we endorse that garbage. SMS is the epitome of terrible fan character cringe, and this is why he's so reviled by such a large percentage of the community.
Essentially, SMS is this old video in a nutshell:
https://www.youtube.com/watch?v=QPtit-yjy34