So guys, I have a story. I used to play Magic the Gathering and hang out at the store. On Friday nights there was a tournament, but there was a barrier to entry, I didn't have a deck. So a friend, who I had actually met like a week prior, loaned me his 200$ Jund deck and took it back after the tournament. I made sixth!.
My point is that if you play competitive smash, and you don't have a 3DS, you still KNOW someone who has a 3ds. Making miis usable by 3DS transfer only (no miimaker), should be a reasonably surmountable barrier to entry. Just borrow a 3DS or 2DS from a buddy.
The mii fighters overall seem like low-mid tier characters, so I think it's undesirable to lower their usage rate by nerfing them to standard miis only.