Someone who slavishly follows fashion trends.
Fashion victims have been around for longer than you might think.
This term came into prominence in the 1970s and 80s. It is an older phrase than that though and was in common use in the 1930s; for example, this piece from the Utah newspaper The Ogden Standard-Examiner, July 1932:
"Hips must still be hide their humps beneath elastic metal stays, but the fashion victim will be permitted sufficient expansion of the diaphragm to draw a reasonable number of good breaths per hour."
See other phrases that were coined in the USA.