Maybe not, but there will be a balance patch, and it will likely reflect whoever complained about things the loudest.
And competitive players are the only ones really available to cater to for Mewtwo and Lucas and beyond. Players who don't care have probably already put the game down after emulating their Mario vs Sonic fantasies.
Either way, if anyone balances any game, they'd need a reason to. Which means some sort of feedback, and some sort of proof. At the very least there will be balance feedback based off of Apex, and any other Nintendo-sponsored event since.
I anticipate Apex crowd reaction and finalists being large dictators of the patch. Diddy is almost guaranteed to have something done to him, or many people may very well just give up on the game (what's the point in hoping?). Rosalina may receive some tweaking due to the Pacman match-up, and her gravitational pull may be nerfed in some way, either by range or by endlag.
Other characters are up in the air. For all we know, Nintendo may be blissfully unaware of current tiers and nobody will be buffed since there's no evidence that they need buffs. The catch 22 is nobody will play as weaker characters, and therefore, visually, there's no evidence these characters are weaker.
Of course this is really dumb speculation. It is the bare minimum, though.