Let's face it: out of all the playable Pokémon characters in Smash Bros., Mewtwo has garnered the most positive reception.

Lots of people like Pikachu but its status as franchise mascot also leaves it in a similar spot as Mario where it's considered too basic for many people to be interested. Jigglypuff is only liked by ironyfags and memers. Everyone except furries dislikes Lucario because it's been shilled to hell and back by Game Freak and TPCI. Greninja was FOTM when Gen 6 was new and now there's nobody left with any interest, plus the ridiculous Ash-Greninja form led some people to actively dislike it. Incineroar speaks for itself with the amount of backlash it has generated here and elsewhere, not even as a confirmed character but just for the possibility of being one, and simply in general.

Pokémon Trainer is probably going to give Mewtwo the closest thing to a run for its money with his unique three-for-the-price-of-one playstyle and three characters that can attract fans of each of them, but he's just not in the same league. People will always like and remember Mewtwo for the role it played in the first Pokémon movie and its prelude, and for being just about the only Pokémon out there that is an actual character with a relatively complex backstory and personality for the standards of its own franchise. The only people that are opposed to Mewtwo are those who are mad assblasted about Gen 1 and will denounce anything from it on principle, and those who got annoyed by Mewtwo fans due to their very justified outcry when it was cut from Brawl, and these are hardly valid reasons as they aren't about Mewtwo itself.