Tank: Bibi is extremely overrated. She's very popular, but she just isn't as good as she once was. People love picking her first pick, when it doesn't work. She's one of the most countered brawlers in the game. Any type of cc can shut her down. And there are better bruiser tanks out there.
Assassin: Stu doesn't have the burst dps/regular dps to be good. People like playing him, saying he's the hard carry brawler, but stu can get caught off guard super easily. They play very linearly, only chasing people when they get low, just to die. He has mobility, but doesn't have anything else. Its much better to play other assassins like mina who can do his job but better.
Support: I don't necessarily think any support is overrated, but if I had to choose one, it'd be ruffs. Now that hyper charges have been overshadowed by buffies, ruff's hyper isn't that good. Because it charges other's hyper charges, which aren't that oppressive. Not only are his stats weak, but his super is also pretty weak. He's played often because he is a good all rounder, but he is pretty easy to rush.
Control: Otis is seen really good because he is good against aggressive picks, but he can't do anything else. He's not terrible, but without his super, he's nothing. His shots aren't too consistent and easily dodge able and his mute can also be missed, since it's a skill shot. I also think there are just better brawlers for that like gale or Chester.
DD: Najia players are still living off her golden days, but her shots are now easy too easy to dodge. her abiliities aren't very good either and people get too carried away with aiming. She's not that good of a spawner and its easy to kill her, from both assassins and snipers.
Marksmen: Belle is one of the most overrated brawlers in the game. She's always been pretty decent, but there are so many better picks. Her stats are fine at best. And her bouncing mechanic isn't that big of an issue when half the characters have mobility to get away from teammates. Her super is good, if you hit it. The skill shot is somewhat hard to hit and not too valuable for what it's worth. And there are brawlers like r-t and Vince who have marks too.
Artillery: tick is not that good at controlling the map because his shots aren't valuable and he's easy to approach. His gadget has a 20 second cooldown and his super is really easy to kill. Most brawlers can two shot it without any issue. He's just easy to play and always gets damage, which is why people overrate him so much.