I would sooner play as customizable Primids than Miis, to be honest.
Seriously, they have no background. No story. No goals, accomplishments, purpose. They are literally just avatars for humans in real life, not characters. We already have Wii Fit Trainer to represent the casual demographic of Nintendo, and Game & Watch and Villager to represent the "pulling out everything out of hammerspace" niche. You can't give Miis any sort of credibility compared to characters that have saved worlds, enslaved dimensions, and brought championship status upon themselves. Even WFT, an individual, has credibility in the form of "making millions more fit" as that's a consistent canon to her history and purpose. Miis do not have a single notable role to speak of beyond linking the player to the game, something Link already does in name and history.
Customizable Primids at least work within the context of Smash Bros. Adding Miis would be a giant ass-pull to appeal to masses that would rather play as themselves than someone else in a fighting game all about Nintendo characters. There is literally no reason to add Miis besides to detract players from getting familiar with other characters, the complete antithesis of Smash Bros.'s purpose.