mixed bag vs aggregation

mixed bag and aggregation both are nouns.

Word	Noun	Adjective	Verb	Adverb
mixed bag	Yes	No	No	No
aggregation	Yes	No	No	No

As nouns, aggregation is a hypernym of mixed bag; that is, aggregation is a word with a broader meaning than mixed bag:

Other hypernyms of mixed bag include accumulation, assemblage, collection.

mixed bag (noun)	aggregation (noun)
a collection containing a variety of sorts of things	the act of gathering something together
	several things grouped together or considered as a whole