mother figure vs adult female

mother figure and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
mother figure	Yes	No	No	No
adult female	Yes	No	No	No

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

mother figure: a woman who evokes the feelings usually reserved for a mother
adult female: an adult female person (as opposed to a man)

Other hypernyms of mother figure include woman.

mother figure (noun)	adult female (noun)
a woman who evokes the feelings usually reserved for a mother	an adult female person (as opposed to a man)