Baz made this particular Gatsby for this moment. Of course the costume designer reached for current ideals of beauty --because Baz's entire visual language is grounded in this moment. Just look at the size of the "cars."
Baz was paying high homage to Fitzgerald with his numerous a-historical visual features. For the truly timeless works, the setting, costumes, and other affectations can be altered radically and the work stays true. That's what "timeless" is all about. Romeo and Juliet was just as fantastic when set on the Upper West Side and staged as a Broadway musical.
next story: some expert reveals there wasn't Rap music the 20s, either.
to be followed by: African - Americans were treated far worse in Jazz Age Manhattan than suggested by Baz's movie.