I think he's somewhere in the 4-6 range, where top 3 for me is Fox Wolf Diddy in some order (probably that one, Diddy might be better than Wolf, it's hard to tell). I've always thought MK is kind of overrated, although having played him a bit I don't feel quite as strongly about it as I used to; he's probably also in that 4-6 range. Who might be the third character in that range is a bit less clear. Could be Sheik, could be Falcon, could be Wario, could be ROB, could be a few others. I do think Mewtwo's probably the best floaty?
Keep in mind that I largely judge characters based on how many very bad matchups they have. The goal in a tournament is to lose less than two sets (flexible RR pools notwithstanding). If your character has a matchup that you can't realistically expect to win, and someone who plays that character at a high level is near you in bracket, you can't realistically expect to win the tournament unless they get eliminated by other high-level players and you make a pretty dramatic loser's run. So I put a lot of weight on the likelihood of that happening when I judge characters. The larger the tournament, the more players you have to face, and the higher the chance is that this'll be an issue.
Fox has no particularly bad matchups (which I'm defining as "more than mildly disadvantaged"), and as far as I can tell he's the only character (though I don't think that makes him broken). Diddy's worst is supposedly Samus(?); Wolf's worst IMO is Fox, though that's hardly unwinnable (a lot of Wolf players hate Falco too but I don't really mind that one). Having spoken to Mewtwo players before, I'm under the impression they only have major issues with Toon Link, and even that's not unbearable. Can't speak for the others I mentioned above.
Then compare to Bowser, who gets really badly countered by like half a dozen characters and loses moderately to a whole bunch more. He's by far the most extreme example, though. (And of course you still have to sort of respect him; it's not like Melee Kirby where you can snore through the match unless the opponent is noticeably better than you. I feel like he's around Melee Ganon tier, just in a cast that's all stronger than him.)
There's also a metagame factor, where if (for example) Samus suddenly gets really popular for some reason, Diddy Kong becomes less good, despite the fact that no balance patch or technical advancement has occurred. (I'm using the "trying to counter what's popular" definition of metagame and not the "how good people are at the game" definition there.) Or there's the same effect on a geographical level rather than a temporal one: Diddy Kong is better at a local with no Samus mains than a local with 2-3 of them on the PR.
idk tiers are hard maaaaaaan
kk