It gets worse the more they think about it. The Miis would seem to have come straight out of nowhere. They just started showing up at parties, the Olympics, go-karting, baseball, basketball, volleyball, dodgeball, ice hockey, tennis, golf, and now Smash Bros. They never seem to do anything noticeable. They also exist in the Pokémon World, where they run a ranch. And if the Smashers were to take a look at the Miis' own world, what would they find? Going by Wii Sports, Wii Play, Wii Fit, Wii Music, Wii Sports Resort, Wii Fit Plus, Wii Play: Motion, Wii Party, Pilotwings Resort, Nintendo Land, Wii Sports Club, Wii Party U, Wii Fit U, and Tomodachi Life, they all seem to have hobbies that resemble those of the other worlds.
So long story short, the Miis would lack any explainable origin, imitate other beings, and seem to be an invasive species. They could just as well be the Borg.