Is this an animation error, or just bizarre character design?
As mentioned in my comment, this doesn't seem to be an animation error. Aku No Hana is rotoscoped, which is essentially drawing over live action sequences. Here's a shot from before it was animated:



I suppose it was intentional if they had this character dressed up like that. I haven't seen the animated show, but the character definitely doesn't appear in the manga, having read it myself also.

Just a guess, but perhaps it was to exaggurate ugliness of a character because the animators couldn't do it that easily with the technique?

So, yes it does seem to just be bizarre character design. Especially as the character barely appears in the anime and only has a single line.