skin and bones vs mortal

skin and bones and mortal both are nouns.

Word	Noun	Adjective	Verb	Adverb
skin and bones	Yes	No	No	No
mortal	Yes	Yes	No	No

As nouns, mortal is a hypernym of skin and bones; that is, mortal is a word with a broader meaning than skin and bones:

skin and bones: a person who is unusually thin and scrawny
mortal: a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

Other hypernyms of skin and bones include individual, person, somebody, someone, soul.

skin and bones (noun)	mortal (noun)
a person who is unusually thin and scrawny	a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

skin and bones (adjective)	mortal (adjective)
	subject to death
	unrelenting and deadly
	involving loss of divine grace or spiritual death
	causing or capable of causing death