Yes, Mii exists on Switch but are hidden so not used that much, while it was the first thing you have to do on a Wii or a 3DS (yet another wonderful piece of hardware). You really can see no-one care in the Mario Kart lobby. Everyone use default one because I'm sure no-one really know where to create them (I mean, the Settings is a place very few peoples actually goes in). Sad. If an application (with an icon) would have exists, it would have change a little.

My only complait is to not having them used inside games. I loved Mii statues in Mario Kart Wii, or seing them driving cars.

But maybe I should simply admit no-one care about avatars in 2023. On Wii, your Miis stands inside your Wii instance. With Internet peoples create crazy/scary/disturbing (yet wonderful) Miis, but you don't have any value having simple, normal, Miis.

On 3DS you could meet Miis while moving around (I was in a big city and there were many peoples using that feature). I loved this, but I suspect it create a privacy problems : When I moved to a tiny city, I identified the only little girl with this feature and could see it's games. So while digging into anonymous gaming life was awesome and unproblematic, I feel very weird in my second place, and disable the feature.

Miis are something from the past, but man, it was so much better than tiny icons that are actually used nowhere (Smash Ultimate is a few exception, but it's used in few sub menus while I expect it to be show when combat starts).