cover girl vs adult female

cover girl and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
cover girl	Yes	No	No	No
adult female	Yes	No	No	No

As nouns, adult female is a hypernym of cover girl; that is, adult female is a word with a broader meaning than cover girl:

Other hypernyms of cover girl include photographer's model, woman.

cover girl (noun)	adult female (noun)
a very pretty girl who works as a photographer's model	an adult female person (as opposed to a man)