Mythra's appearance as a Spirit in Super Smash Bros. Ultimate had a different outfit with black pantyhose and no cleavage cutout. This was later included in Xenoblade Chronicles's second wave of DLC with the unlockable "Massive Melee Mythra" Aux Core, which changes her appearance when equipped.
This appearance became popular among the internet, and a fair number of fanarts featured this appearance, and most (but not all) are tagged with super smash bros. here.
I think that costume alone does not warrant the super smash bros. tag.
- It is included in the base game.
- It doesn't really have anything to do with Smash.
There's only about 80 images tagged like this; there's no need for a BUR, I can easily do this myself with scripts, but I'd like to hear objections before I do so.