The word “costume” is usually interpreted as “a set of clothes worn by an actor or performer for a particular role”, and is as thus a diminishing term to use to describe certain cultures styles.
With little representation, most people only know a Gothic style as something you dress up as for Halloween or for a music video. And then tend to you stop thinking of those dressed that way as real people. This is a problem shared by many communities, whose particular style may be viewed as rather strange by an at large Western society that dresses in what can only be described as utterly boring sameness.
All this to say; don’t call strangers names in the street.