>>32203838I'm not sure how you come to that conclusion. Something that applies to one Pokemon doesn't necessarily apply to all. There are Pokemon with clothes. Sawk and Throh, Mimikyu, even something from Generation I, Hitmonchan. There's nothing to suggest that's just part of their skin, not clothes.
Unless there's some evidence that suggests Gardevoir's dress is part of its skin, it's all just guesswork. It looks like clothes, so I'm going with clothes. That's my reasoning. There's no evidence either way, so go with whatever you want to believe.