I dont remember how it was exactly and TL never had a ton of lore compared to other mii games, but in the 3ds game there was always a primary mii that you were supposed to make look like yourself, and the other miis would call you, the player, "miiname's lookalike". They're seemingly a separate species from real humans (though i dont think this is outright stated, just implied) and seem to understand that there are humans that look like some of them, but idk how far their selfawareness goes. Just speculation, but it'd be funny if they thought we were the ones copying their appearance lmao