Are we talking about only Fox? I only played that MU once last night against my old tourny team mate/sparring partner Life so I don't have an opinion on it... besides that it's highly likely that Fox still handily beats Mewtwo. the buffs made very little difference aside that he now has to fight Mewtwo like a Peach with bad priority.
after an hour straight of the MU I will hazard that Falcon can still **** all over Mewtwo, only now it's stage dependant, so counterpicking might come into play. oddly I won on every big stage we played on and got bodied on the small stages because Falcon can knee everything Mewtwo tries to space with; tail attacks, Teleport startup, Hover, even Confusion and Disable. on big stages Mewtwo can win and actually probably has a slight advantage because he can zone and counter zone with the Hover and Teleports into well spaced Nairs and Fairs. Winning this MU will require you to make Falcon misstep, and you can do that on bigger stages.
We played a few Ike vs Mewtwo matches and I feel like that might be in Mewtwo's favor. I'll need to play it more before I can really say.
ZZS vs Mewtwo is a battle of attrition, but one that Mewtwo should win more often than not. challenging her mobility is difficult and her nair is a nightmare, but Mewtwo can clank with most of her attacks and force her into a corner. I stopped bothering to try and edgeguard becase of her super slippery recover options, but racking up damage into an onstage kill is inevitible. on both sides actually, because ZZS doesn't really have an iron clad way of edgeguarding Mewtwo other than the Mewtwo missing DI hard.
Roy vs Mewtwo makes me sad. Roy got huge buffs and it shows in this MU. I lost all but the last two matches when I generally win nearly all of them in Melee
These are mostly first impressions and I haven't gotten total control of Hover yet, but I don't forsee current Mewtwo:M being a serious contender in tournament any time soon unless he gets further buffs (meaning i agree with M2k lol). Roy on the other hand... we might be seeing a lot of him.
I'm going to stay in the lab and hope that I discover something that changes my opinion, nothing better to do since Bowser got hit with the nerf bat.