None of those seem like very good reasons to look down them.
Competitively, Miis are much better and way less janky this time around with their readjusted proportions and moveset buffs. The custom moves give them some versatility and variety in their gameplay.
In regards to aesthetics, that is merely subjective. Miis are hardly the ugliest type of character you'll ever see.
The point about the costumes makes me roll my eyes. No one should realistically expect every character they wish for to be in the game. Making a costume out of someone is way to pay homage to them despite the actual character not being in the game. You'd think people would appreciate this gesture, but I suppose when some of them see a Mii wearing a costume of Black Knight, Zero, Tails, Megaman EXE, Monster Hunter, K.K. Slider, etc., they fall to the ground and curse to the heavens wishing that they'd never do that. For them, It's either the real deal or bust. No middle ground in between despite that some of the characters I mentioned earlier are highly improbable. They are so sensitive to that that I'm surprised the real world hasn't eaten them alive.