I don't care if I'm late for the discussion or I start it up again, but the prime reason I'm anti-Mii is because they have potential to outshine the rest of the cast by their superficiality. They're nothing but glorified, template avatars. Given the situation that they're custom characters, it feels forced to invest the main feature of the game to use them. I can just see people flooding the game with their avatars and completely cutting out pieces of the main characters and sticking it onto them. In games where they don't have autonomy, such as Pilotwings Resort or Mario Kart, they're just fine. But there nature doesn't make them fit for an all-star fighting game such as Smash Bros. Not much about them excites me with moveset potential either.
That being said, it's not like they can represent them in other ways like their stage representation for example. For this new Tomodachi Collection stage, imagine seeing Miis saved on your system doing various activities in the background and maybe occasionally interacting with the stage. They could even expand this by taking Miis from StreetPas Mii Plaza, your Miiverse friends, followees, and followers, etc.