maybe rate moves on usefulness then each mii has a certain amount of "points" like in other game where you choose stats
That would be good material for an entirely different game.
The fact is, if you could customize miis, people would figure out 2-3 top tier mii customizations that would be equal to or superior to top tier characters, and Smash brothers, in the competitive field, would turn into a bunch of battles between two miis.
On the other hand, if they made a "mii wars" game that had different physics and a different fighting system than smash but allowed you to customize miis for battles, that would be really cool. I just think it would be quite disappointing to turn smash into that.