Some people like to still insist that ROB isn't top tier or that Wolf still isn't ****ing gross, but I take it the the thread isn't about already well-off characters.
I suppose if anything, I'd say Marth. Most people afaik view him as a mediocre at best character, but I feel like it's an overreaction and we're just repeating what happened in Smash 4 where people for a long time thought Lucina was (after most of the FE cast got buffs anyway) low tier, but then eventually came to see that they weren't that far apart. Or how people used to think Cloud was bad in this game because he wasn't the disgusting god he was in the previous game. Marth is a character design that's proven to fundamentally work--he's been solidly high or top tier in every game he's been in except this one. If it wasn't a fundamentally solid design, Lucy wouldn't be any good. He's just ****ed by the game engine, if I had to hazard a guess, which is why Smash Ultimate Marth is easily the least successful or enticing version of him. Not saying he's still secretly top tier, just not as bad as his (lack of) results might indicate.
Mac is....an interesting case, for lack of better phrasing. Tbh I find him to personally be one of the most unfun characters to play against, and really he shouldn't have been brought into the series, not in this form anyway. He's just a horribly polarized character that was designed to not be able to play half the game and then was "compensated" for it by giving him bull**** like armored smashes and KO Punch and **** like that.