man in the street vs commoner

man in the street and commoner both are nouns.

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

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

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

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