Mewtwo's Jab setups, in particular, are incredibly solid. Off of Jab 1, he can (of course) go for his Jab combo for safeness and it has pretty solid range in front of him (but not above him). He also has Confusion which can potentially lead into OTHER things, namely Shadowball/Disable/Utilt/Usmash. Additionally, Jab 1 can also potentially lead into a Uthrow for an easy kill. Then there's Fair and Shadowball for other kill moves. LASTLY, Jab 1 can also lead into Disable against an unsuspecting opponent, which can make them eat a fully charged Smash attack.
Confusion is surprisingly an incredibly potent special this time around. Including the setups mentioned earlier, it is the only reflector in the game that is also a grab as well. And it is quite fast too. And it works kind of like how Mario's cape used to work, only better. The endlag is a bit punishable but it is really solid otherwise. Mewtwo's specials in general are fantastic, although Disable is slightly niche but it still has its uses.
His problems are his low health, and being vulnerable to opponents directly below him or behind him. When above an opponent his landing options are kind of small. Nair and maybe Dair. Most of the time I find it best to teleport away and stick to the ground. And because Dsmash doesn't hit behind him, and Bair is slow on startup, he has to rely on Nair or other tactics when somebody catches him from behind. And his Smashes, while strong, aren't really too useful besides Usmash. Usmash is great for catching larger opponents spotdodging and it is VERY strong, but it'll probably lose a lot of potency on smaller characters.
Small, fast characters are probably going to be his bane. But he can probably handle most other characters with the right amount of skill. He is very high risk, high reward. Probably upper mid tier most likely.
Lastly, Dtilt is godlike, like
TTTTTsd said. Use it. Easily one of the best Dtilts in the entire game.