Mewtwo seems pretty clearly to be high tier, at least. He just has everything you need; very strong neutral, explosive kill power, long and hard to intercept recovery with mixups, the best fair in the universe, and high ground and air speed. His weaknesses are relevant, and play into the fact that many players see their main going even with him. The potential to get killed early is more or less always present. However, I think his MU spread is very good, and he also has a niche as a powerful anti-zoner.
He has already proven to be capable of winning large events more or less on his own, and he is a relevant threat. I have heard arguments that he goes even with or has a slight edge on Cloud or Bayonetta. However, there is also the fact that by design, things can quickly turn sour for him. Such a character should exist alongside others of a like "power level"; alongside Pikachu, Ness, Meta Knight, Ryu, and Toon Link. Around top 10 but not top 5. I don't have a well structured opinion of the meta at the moment, so I cannot place him more specifically. I do have a very high opinion of Mewtwo, though.