Minecraft was an indie game and then it was bought out for Billions of dollars. and is owned by one of the largest gaming companies in history. That is not Indie.
Depending on your definition of Indie, I guess you can consider NMH as Indie.
But I don't think they will or won't add a Mii costume based on what Genre they are.
It's not Indie now, but it was in it's origin, and considering Mojiang AB is listed (which is their OG Name I beliieve), I think the team know that
In any case, I don't think they are Adding costumes based on genre, but I think there's a good possibility they they Position the mii costumes they are making to the best spot where they make Sense, in terms of Theme. That theme might be based on Genre, like with Fighting games. It might also be based on Company, Requested characters, Weapon Triangle (Byleth's), etc
But that's just a Theory, and looking heavily into it. If Fortnite is a Costume, that doesn't automatically mean we get an FPS Character, but if FPS Character is in Pass, That's perfect place to put Fortnite, even if Fortnite is 3rd person