The Miis were never given a real chance in 4. The politics of customs are something I don't think we really need to go through right now (it's not a topic that brings me much joy), but when the custom move ban happened, it basically removed any possible future from the Miis. Different tournaments interpreted it differently, but at most events either the Miis were altogether banned or limited to medium size 1111 which made Swordfighter a typical low tier and Brawler and Gunner probably the two worst characters in the game. This meant that if you wanted to main one, at most events you were getting a bad character and even worse unlike maining other low tiers you couldn't even trust you'd consistently have your character at every tournament (like as bad as Ganon was, if you wanted to main him, there was at least a 0% chance any tournament would prevent you from doing so!). Investing into one of them is a high risk move with no real reward. It shouldn't be a surprise the outcome from there was basically literally no one playing them.
In Ultimate things can easily be different. As long as the menu for Miis is non-stupid, there's no reason they wouldn't be fully allowed this time, and any character fully allowed will be approximately equally considered by the competitive scene as any other. No one really cares if Miis are stupid or if other characters are cooler or more beloved outside of Smash; when you're playing seriously, characters are functions and it doesn't matter at all what they represent other than those functions. The Miis represent three characters, and as long as the dynamics of selecting them work out well, the competitive community will give them the same type of shot as any other characters. If they're good, they'll see a lot of play. If they're bad, they won't see much play but won't see particularly less play than other bad characters. It's just the natural way of things, and all else being equal, having multiple sets of specials is just an increased probability that at least one Mii configuration will be good enough to be worth using.