Let's see... I think this character has some very bad matchups, but some winnable ones. He still has some gaps in his game that's ultimately going to hold him back if they aren't addressed in future patches.
-Mewtwo's hitboxes are janky and have some odd deadzones in them. Also, the way they interact with other character's hurtboxes at times, don't make any sense. I feel like I'm flipping a coin if a move is going to land or not- it doesn't seem consistent all the time for some reason. I also notice this when fighting against a Mewtwo with my main (Sonic.) His hitboxes feel rushed and this is a shame, he would be much better if this wasn't the case.
-Another issue I see with Mewtwo, is one of his best moves- n-air... it seems random what side of Mewtwo your opponent is going to land after connecting the hits- making it hard to follow-up/combo. For a contrast example, with Sonic at low %, I can just short hop n-air to f-tilt all day for a free 23%. It's not like that with Mewtwo.
-Mewtwo's worst matchups are against characters with better framedata- or quicker pokes and jabs. Mewtwo's game is obviously spacing, but his mobility is a bit trash (why does he have so much ending lag out of a dash) and why is he so light? He weighs ALOT in the Pokemon games iirc lol. Sheik is HORRIBLE for Mewtwo. Upclose, Mewtwo has no options for her jab.
I feel a little disappointed, I wanted to second this character if he was viable. Mewtwo hits like a truck and has some great KO options but his hitboxes, inconsistency, and no options up close vs characters with fast jabs (read: many top tiers) are going to hold him back unless some kind of great tech is discovered for him.
tl;dr: this ain't no r/b/y Mewtwo where you could use amnesia turn 1 and blow up most of the metagame with psychic/ice beam
The only moves with "janky hitboxes" are utilt and uair, while bair does rather unfortunately extend his hurtbox.
Hmm... now that I think about it, if utilt had its old Melee hitbox he could be almost unapproachable.
Opponents changing sides during Nair is only really a problem if you land during it (and even then only if they're behind)
or when trying to gimp someone with it and send them towards the stage. Even then you can manipulate this as long as
you can keep Mewtwo on one side of the opponent during the move.
Top tiers have a lot of fast moving characters, and they are his most difficult matchups because his moveset
is much better suited to keeping characters out in the first place rather than trying to repel or turn the tables on
those who have hit him already. He's kind of like a heavy in that regard. Against fast characters with fast options it's much more difficult to do that consistently, but it's certainly not impossible for him to cope with jabs when his own is fairly fast
and has more range than it appears. It even has the slight benefit of being almost visually identical to his grab.
Also Sheik's jab is hardly anything special, if you're having big issues with it you're probably playing Sheik too scared.
I know that her fair commands a lot of respect, but if you give her too much she can just go ham on your shield.
Why do people always turn to official Pokedex descriptions (which can be pretty wack themselves... did you know Joltik
is small enough to fit in your hand?) to complain about Mewtwo's weight? Nobody does that with Charizard who should
supposedly be lighter, but he gets a pass because heavy=good. Weight in this game can be compared to stamina
in other fighters, and Mewtwo has little of it because of the multitude of ways he can kill. Powerful smashes (including a dsmash
unpunishable on shield, and a frame 5 Usmash with good reach), two killing throws, killing projectile, reflector,
deadly edgeguarding, having a swift and powerful shadow claw (also potent as an air punish), strong spike, nair that
can drag people to their doom, and bair that can sometimes Wall of Pain, and he can go really deep to do it on top of all
that. With how lethal Mewtwo can be if played well, it's only fair that he goes down pretty quickly if he's hit himself.
At least, that's how I see it now.