No kidding.
That was part of the impetus behind including Miis in the first place, if the old digital event announcement is to be believed.
I've noticed a couple times when Sakurai has been asked about characters like Segata Sanshiro or Reggie on air or in an interview, he sounds a bit disappointed that he has to explain that Mii Fighters are there to serve that purpose. I think Mii Fighters aren't properly appreciated by players, and on one hand I get that, since they aren't useable online in the modes most people play. But on the other hand, there's a lot of work out into them, and I think they turned out quite nice for what they are.
Frankly, if a Mii Fighters online mode, or perhaps a Customization-On online mode has been in the game, some of the more 'out there' requests would vanish. More importantly, it would make it easier to appreciate some of the more interesting features in the game, even if said metagame wasn't completely balanced.