I'd believe it with Mewtwo. That character is damn scary now. It is just hard to be consistent when you are super light, especially when you have really committal moves like nair, confusion, and his pretty bad grab.
That said, he does have a lot of non-committal spacing moves. Fair freakin kills all the time, regardless of how stale it is. That move alone could make some character's neutral, but he also gets a nair that nets him followups up until it can straight up give him a kill. It doesn't seem like Mewtwo fails often so much as the other player makes some good reads that make Mewtwo regret being aggressive.
I still think Greninja beats Mewtwo, the slow double jump away and the lightness together make usmash terrifying, and shurikans can actually compete with shadow ball for forcing approaches, if you are clever with your charge