poor fish vs pudding head

poor fish and pudding head both are nouns.

Word	Noun	Adjective	Verb	Adverb
poor fish	Yes	No	No	No
pudding head	Yes	No	No	No

As nouns, poor fish and pudding head are synonyms defined as:

Other synonyms of poor fish include dolt, dullard, pillock, pudden-head, stupe, stupid, stupid person.

poor fish (noun)	pudding head (noun)
a person who is not very bright	a person who is not very bright