Whether official alternate costumes should be general tags, character tags, or not tagged at all is one of the more perplexingly inconsistent standards of the site (just look at Fate/Grand Order vs Fire Emblem Heroes).
That issue has just spread into Xenoblade Chronicles 2 with the addition of the Mythra (Massive Melee) (Xenoblade) character tag shortly after the Disguised Pyra (Xenoblade) general tag.
Personally, I think it's kinda dumb to have an entire character tag for what is literally nothing more than just a pair of panyhose and a sewn-up cleavage cutout, especially when we have no tags for their swimsuit costumes. In any case, it should probably be standardized.