... but upon looking it up, prosthetic does seem to have a broader meaning than I thought.

Functionality doesn't seem to be necessary, at least for this definition.

pros·thet·ics
/präsˈTHedik/
adjective: prosthetic

1.
denoting an artificial body part, such as a limb, a heart, or a breast implant.
"he wears prosthetic legs because his own were amputated"
denoting a material designed to alter a person's physical appearance temporarily.
"an actor transformed by layers of prosthetic makeup"



cos·met·ic
/käzˈmedik/
adjective: cosmetic

involving or relating to treatment intended to restore or improve a person's appearance.