I main Mewtwo, so I have experience with how viable he can be in SSBU.
For one, I consider him to be around mid tier to high mid. "Why?" you may ask. Well, for a number of reasons.
First and foremost, his tail hurtbox. The one thing that hinderences him from being in a better placing in a tier list. It's basically like in Melee, and that's not a good thing. Even Smash 4 lessenes the tail hurtbox slightly, but Ultimate brings it back. I'm not sure why they even went with that decision, but it still bothers me the most.
He does also have some other slight disadvantages, such as being undeniably light, and having a slow, floaty double jump. Well, I've grown used to those. The double jump I've mastered with ease since I also mained him in Smash 4. And his light weight... well, it still does get me irked. But not as much as the tail hurtbox.
But for some praise for this character, he does have a good number of options. His setups still do a good job, even though people could argue about that as well. And his kill confirms are perfect, and his power in general. Let's not forget that he also has a phenomenal recovery, with Teleport going very high, and the aforementioned floaty double jump also providing some help.
Finally, Mewtwo has received numerous buffs throughout patches so far. The best would have to be version 8, when they buffed Shadow Ball, Teleport, and his Down Throw. However, I think he's still gonna stay in a mix between mid and high mid tier as long as the tail hurtbox is still a thing.
But what are your opinions on this character? Be sure to let me know.