kettle of fish vs mess

kettle of fish and mess both are nouns.

Word	Noun	Adjective	Verb	Adverb
kettle of fish	Yes	No	No	No
mess	Yes	No	Yes	No

As nouns, kettle of fish and mess are synonyms defined as:

Other synonyms of kettle of fish include fix, hole, jam, muddle, pickle.

kettle of fish (noun)	mess (noun)
informal terms for a difficult situation	a (large) military dining room where service personnel eat or relax
	a meal eaten in a mess hall by service personnel
	soft semiliquid food
	(often followed by `of') a large number or amount or extent
	a state of confusion and disorderliness
	informal terms for a difficult situation

kettle of fish (verb)	mess (verb)
	make a mess of or create disorder in
	eat in a mess hall