Think about the fact that this exact size has been passed through thousands of hands, trades, battles, and childhood afternoons that probably smelled like old school desks and cheap snacks
Any "enters" abilities of the copied permanent will trigger when the token enters
While many of those are not so important late game they are still things you want to take on curve
Making matters worse, to support backup commander Zaxara, the Exemplary, the deck also has some random X-spells that are not especially playable as well as creatures like Hungering Hydra and Vastwood Hydra, which don't do much to support the primary mutate theme (other than possibly add a creature with some +1/+1 counters to the battlefield to mutate on)
Gardevoir has one main weakness of its own, however, and thats the fact that it is a Stage 2 Pokemon: if you cannot get a Raltz and a Krilia, as well as a Gardevoir, early in the match, Mewtwo EXs big problems will begin to bleed through and open an opportunity for your opponents to punish
Card types such as "artifact" can't be chosen