It's long been a joke that the formula for generating aliens on Star Trek is to give a bipedal humanoid a weird prosthetic forehead. And non-human ears.
Star Trek: The Original Series, very much a product of the 60s, followed another formula for the fashions of futurity and alienation. For women, it was bullet bras, beehive hairdos, hemlines all the way to the Orion belt, and miles of pastel-colored chiffon caftans. For men it was varying-lengths of bell bottom pants, silvery tunic tops, and silly hats.
Slap on a prosthetic forehead and some rubber ears… Aliens.
See more costumes in this MeTV piece. And this piece on ScreenRants.