man in the street vs Joe Blow

man in the street and Joe Blow both are nouns.

Word	Noun	Adjective	Verb	Adverb
man in the street	Yes	No	No	No
Joe Blow	Yes	No	No	No

As nouns, man in the street and Joe Blow are synonyms defined as:

Other synonyms of man in the street include Joe Bloggs, John Doe.

man in the street (noun)	Joe Blow (noun)
a hypothetical average man	a hypothetical average man