For all of those saying "it is only day [X]", allow me to say this:
During the smash fest, (my first time playing Smash Wii U), I picked up Mario and beat my opponents, scoring 5 kills and dying 0 times.
During the first time I played Smash 3DS, I picked up Wii Fit Trainer and obliterated people on For Glory, never even losing once. Wii Fit is now one of my mains.
During the first time I played Smash Wii U (post purchase), I picked up Zelda, went online, won some matches on For Glory, and didn't lose any.
During the first time I played Mewtwo, I went online, won 4 matches, and then got my a**** kicked for the rest of them. I fought 12 matches total. I lost eight times, and I have won online tournaments before, I have fought pros and eliminated them, yet Mewtwo just scored me some losses. So no, he isn't "good" he's pathetic, and I'm only saying this because I like Mewtwo, and being the type of person I am, when something I like falters in quality, I will bash it (and see if I can make the thing better).