Sankaku Channel/fashionplate

A fashion plate is an illustration (a plate) demonstrating the highlights of fashionable styles of clothing. Fashion plates are not depictions of specific people, but are instead generalized portraits, meant only to dictate the style of clothes that a tailor, dressmaker, or store could make or sell, or to show how different materials could be made up into clothes.

Used figuratively, as is most often the case, the term is a reference to a person whose dress conforms to the latest fashions.