Miis were the one thing I was really hoping I was wrong about pre-Smash 4. I knew they were coming, but I prayed they weren't because I HATE avatars.
The Mii Fighter costumes aren't supposed to be of those characters, they're supposed to be your Mii, dressed as those characters. So Heihachi isn't in the game, but you can dress as him.
I just think that in a game that has many of the greatest characters to grace any kind of media (not just video games); Mario, DK, Link, Pikachu, Ryu, Sonic, Pac-Man etc, why would I want to play as little custom men (or women)?
If I had been implementing Mii's myself, I would have given them a bogstandard moveset (with customs like other characters got), allow you to customize their face/hair/costume etc but they'd just be called "Mii", and they'd be usable in all modes. No 3 moveset sillyness, make it what is essentially the "Mii" of Smash Bros.