I think customization of movesets is fine. The way it looks now, that is. I also think miis should be allowed, along with palutina.
I agree with Kalierdarke: Which moves are being used should be shown.
The reason why I think this, exactly, is because customization adds a whole new aspect. Instead of having all those tiers, in which the lower ones are pretty much screwed because they have very hard time beating the higher ones, customization allows players to also pick lower tiers. Possibly, with the customization, it can be on equal footing.
Also, people now are more likely to pick chars as well they didn't like much before. I, myself, am not a big fan of mario. However, with a slow moving ball of fire, I sure want to try him.
However, I do think people should be on the look out for 'over powered' customizations. When it comes to characters, I don't think chars should be banned too soon. But, when it customization, I think every move that makes a char too strong, should be banned. But, as long as move just slightly changes gameplay so they are used more often, it's in my opinion a definite yes.
About the gear/stat thing: I don't think this should be allowed. Whereas customization of movesets only change a particular move, stat changes change a char in it's entirely. I don't think stat changes would make a char OP (keep in mind, it increases one stat but decreases another, so you could go for a fast bowser, but that would be at the cost of damage, so the result is somewhat the same), but, the changes would make it very hard to work with. You can't really counter a char if he can customize it's stat so that the very reason why you counter him is being changed. That way it would be a battle of 'customization', by constantly changing stats so you'd still be countering. At least, that's what I think. Making it very different from moveset customization, because if it's either a ball of fire or a ball of electricity... well, ya know.
edit: hold on, stats can be picked when making the MII's? I didn't know that. In that case, I completely agree with kalierdarke. There should be just some 'guest mii's' that can be used here with average stats. If that isn't possible, then I also think they should be banned.