It's pretty early, day 2, but it's already kinda clear that Mewtwo is a high-risk high-reward character. Crazy up-smash, a kill throw, a reflector/command grab, disable has always been awesome, spike, great projectile, but floaty, light, and tall. Somewhat of a...more consistent Rosalina? No need to worry about a Luma. And instead of absorbing projectiles like her he reflects them, which is: high-risk high-reward.
I think he's going to struggle with characters that have pretty safe tools that can slowly rack up damage like R.O.B. and Villager. He has tools to deal with this like his reflector, but both those characters have a reflector and pocket respectively. High-risk, high-reward. Really feel these MU's will be Mewtwo's most demanding.
Mewtwo should have a pretty easy time with characters that need to approach like DK, Falcon, and Sanic. Those characters don't have projectiles or responses to projectiles and are pretty much just melee, up-close fighters. Mewtwo should be careful, but with enough awareness should easily have enough control to win the match. Mewtwo just sorta has all the tools he needs there with much less risk.