Highly unlikely that Mewtwo will get a nerf in upcoming patches. I don't expect any changes at all. I's very clearly they are buffing him towards a specific design (fast movement speed, high kill power, fragile) and I don't see them going back on that because an elite player won a tournament with him. He doesn't have any overpowered or spammable tools. It's hard to point to any part of his kit which is overtuned or broken, and he already has considerable weaknesses.
The balance team has not been acting randomly for a long time. They've gotten better and better at balancing as the game has progressed, 1.1.3 - 1.1.5 have honestly been pretty on point. They have a very specific agenda and honestly they've been doing a great job. They saved us from Diddy, they saved us from Sheik... they pay attention. Right now, they are trying to bring down historic top tiers who have dominated the game for a long time (Shiek, ZSS), balance DLC characters who haven't yet been tuned (Cloud, Bayo) and bring up characters who previously had serious design problems (Mewtwo, Zelda, Ganondorf, Samus, Charizard) and remove cheese they don't want in the game (MK lost his 0-to death).
I think the devs are pretty happy with where he is and how he plays. He has a coherent design, and no particularly cheesy buttons or kill combos. In the next patch (if there is one), expect further touch ups to recent DLC characters, large buffs to a couple more low tiers who need it, and some top tier nerfs (My money is on Bayo and Rosa). Mewtwo won't get touched, I'll bet on it.