I'd say in this case, what you deem 'other people's excesses' are the reason she has this dress on. I don't think it's about whether the reader personally finds the dress itself tasteful or not. Obviously, taste is subjective. The thing about the story, though, is that she's in disguise at a party where everyone's in similar attire. And Helena acknowledges this herself. She's dressed exactly right for the part she's playing.
The reason I asked if you'd read the whole comic is that the dress and decision of the dress and other related clothing points are addressed in the story--beyond these few selected scans.