I believe Mewtwo is more than viable now. What do we have in total?
Great dash speed + great air mobility + great recovery
Aireal moves with low landing lag or are easily autocancelable combined with all of them killing in some fashion
An absolutely amazing projectile that is overall extremely underutilized to a terrible degree as it not only can kill, but it covers ledge options, can lead into grabs, can go thru every projectile in the game except for very few of them (notably loid rockey) and has a huge hitbox. Shadowball is amazing but underused by a majority of players. Oh and it breaks shields (FCSB + Dash attack)
We have a stun move that again is underused as I think a lot of players consider it useless but really it stuns them for enough time for us to land several powerful punishes from.
Theres more and I could go on but mewtwo is light, and awkwardly large so he isnt hard to hit. But the thing is you can survive for quite a while with optimal DI, and if youre playing Mewtwo well youll be smart, patient, and defensive, and punishing when necessary anyways. Not running in whilly nilly.
Mewtwo is a perfectly viable character in several regards. I think he'll have the same issue as always in the fact that he wont have many players representing him.