Really, I see a lot of care for them too.
It's just not on your end. People like Mii costumes. There's discussion every time a DLC character comes up. The Shovel Knight fanbase actually is proud to have a Shovel Knight costume. Not every costume is remotely not-cared about as you put it. Rex is like... kind of the only one people actively dislike as a whole, due to why it's in. And that's the only one with bad context towards it. The rest? They're all just good stuff to have fun and give references to other characters. And the only ones kind of unpopular generally beyond that are some of the hats that feel like wearing the skin of a person compared to being actually decent as a costume(K. Rool, Isabelle, etc.) Those do actually look bad. The rest, not so much. Geno was an amazing costume, as is actually most of them. Slime Hat is one of the few hat-only that was met well, mostly due to Hero already being there so it was the only proper way to represent the blob outside of a playable character, while still being high quality(well, all the costumes are anyway) and having great jiggle physics, showing off some cool moments in gaming.
I see tons of shots of Mii Fighters over and over again online. Clearly they're cared about a lot because it's talked about a lot. So I really don't see what you're saying is actually a thing. Mii Fighters were not heavily played online in Smash 4 due to being unable to do in certain kind of matches. I don't even remember if you can fully play them in all online modes in Ultimate either. Can somebody confirm that? It's also kind of hard to consistently figure out a build to play online in various matches due to having custom moves compared to others who have a clear moveset. When you actually are being ranked for your performance, you need a solid character you can practice with, not taking the time to figure out a perfect set up that may or may not work.
They're complicated characters and were even banned in tourneys due to not having a core moveset. Of course they aren't being seen as played much. They're banned in some cases.