man in the street vs common man

man in the street and common man both are nouns.

Word	Noun	Adjective	Verb	Adverb
man in the street	Yes	No	No	No
common man	Yes	No	No	No

As nouns, common man is a hypernym of man in the street; that is, common man is a word with a broader meaning than man in the street:

Other hypernyms of man in the street include common person, commoner.

man in the street (noun)	common man (noun)
a hypothetical average man	a person who holds no title