As in she wears dark clothes, is calm, doesn't bother keepin' it fake to match the mood, and is a little too cool for whatever's happening around her, which gets translated as "downer" because it's literally ダウナー系 or
daunaa-kei in japanese, with the etymology taking many, many steps to basically imply they have the personality of someone "on downers" or has that effect on the atmosphere or people around them.
Or at least that's my read on a subculture/style that I find very hard to translate to american culture because it's too shallow to line up with equivalents like "moody artist" or "poetic goth girl" or something. Somebody please come do a better job.