muttonhead vs stupid person

muttonhead and stupid person both are nouns.

Word	Noun	Adjective	Verb	Adverb
muttonhead	Yes	No	No	No
stupid person	Yes	No	No	No

As nouns, stupid person is a hypernym of muttonhead; that is, stupid person is a word with a broader meaning than muttonhead:

muttonhead: a stupid person; these words are used to express a low opinion of someone's intelligence
stupid person: a person who is not very bright

Other hypernyms of muttonhead include dolt, dullard, pillock, poor fish, pudden-head, pudding head, stupe, stupid.

muttonhead (noun)	stupid person (noun)
a stupid person; these words are used to express a low opinion of someone's intelligence	a person who is not very bright