>It's just a plain generic human female.
You kinda answer your own question. Not every waifu needs to be spectacular or something. Mii gunner is simple which can make her appealing on certain aspects.

I suppose all is a matter of perspective, every character has something to offer: there are Peach followers, there are Samus followers, there are Corrin followers, and so much more! Each one for their own reasons and appeals. And with Mii Gunner,being a "blank" character, it's fun to give her a personality, give it a mold, have the community participate and contibuting in forming their own image of her, she can be whatever you want! Also her design is simplistic and cute (both versions) that it can appeal to certain people, the posibilities are endles! (even the lewd ones hahaha).

At the end, all waifus are derivatives, ones can be seen more obvious to others of which one Nintendo is trying to appeal to; but all in all, no Nintendo character is left behind, even Mii fighters :)!