I'd rather say his attacks aren't fast enough. His power is great and not stupidly OP(ie ZSS's Up B), however his attack speed is quite slow in some areas that matter. Mainly Smash attacks(not Usmash), Ftilt, specials, and aerials. His Utilt and Dtilt are fine speed wise, however the inconsistent hitboxes are small issues. If Mewtwo's attacks were cleaned up, I"m sure his performance overall would be much better even with the same weight and power.
I should have been more clear, but when I said "strongest", I meant in terms of offensive capability.
Jigglypuff and G&W aren't that strong either (rests, judge 9s and oil panics aside), but they do have mobility
(and mad disjoints in G&W's case). Mewtwo trades that (but not too much) for all-round power.
Which sucks because that mobility in the air is a defensive option for Jiggs and G&W. Mewtwo doesn't have those defensive options--he's actually easier to kill than Jigglypuff because of his size and his lack of mobility.
This is the thing--I'm tired of hearing this "glass cannon" BS. Him hitting hard does not justify him being the second lightest character in the game. First of all, he's not the strongest or second strongest character in the game. And the people who are aren't the first and third lightest characters in the game. Besides being tossed around at 60% like nothing, he already has weird hitboxes, a slow dash attack, bad grab range, unsafe kill options, a down smash that goes in only one direction, etc.
I mean, if you want to talk about pure strength, only one of his smash attacks kills at 90 or whatever.
There is no way you can justify him being that light when you have dudes like Captain Falcon out here, a dude who's above average in weight who has the third fastest food speed in the game, a grab range that so far outclasses even the next highest grab range that it should be in a "SSS" tier all by itself, 8 reliable kill moves (FSmash, DSmash, USmash, DownB, OverB, DAir, FAir, DTilt), two with supreme range, moves that combo into themselves very easily, etc.
And Captain Falcon isn't the lightest character in the game, but if weight classes are based on offensive potential, he'd be K.O'd at 50. But instead, Mewtwo, a guy who needed a weight
buff in Melee instead of a nerf like he got in SSB4, is gonna set the standard for this new "weight is determined by how hard your character hits" BS, they're setting a bad precedent.
Mewtwo can hardly even compete with the other villains in the game--whom also happen to share his size--because of his weight. It's absolutely insane. It's horrible judgement of balancing imo.
Like they really are giving him the Little Mac treatment. You know how he is the absolute best in the game on the ground, but at the expense of absurdly bad recovery and aerial ability? Well Mewtwo isn't even the strongest or most offensively capable character in the game, but he gets absurdly bad weight anyway.
Sakurai really must've thought Mewtwo was going to shoot straight to S-tier in their testing if they really thought the way to make him "fair" and "balanced" is to make him so extremely vulnerable. And that was a mistake on their part.