I agree it is annoying when any racer fills the map with stuff (Mike and Elizabeth is usually who I get trapped in a lobby of 5+, both just as bad). The only thing is her furniture is literally her only comeback mechanic. She probably takes stuns the hardest in the game, if not the most then she's at least just up there. I think the issue that's bigger than if Belle is OP is that, in trying to combat people only playing Monster's Inc characters, they really only widened the pool of level 30+ 3 star+ characters a little bit. Once there are more people who get there with Mickey and Friends, Hercules, and Figment (and honestly really the entire roster, because despite what this Discord makes it look like, I don't think that many people bought more than 1 Founder's Pack), I think it'll be shaken up a little bit.
Editing because I can't make a new post for an hour: The issue really is not Belle. It's just people chose Belle and now that's the highest thing they have. If they do anything to Belle and people level up other stuff, she'll be the worst in the game. All we can do is be patient and hope we get some events to level Mickey 's crew etc.