“Joe Citizen” or “John Citizen” (also “Jane Citizen”) is a generic name for a citizen. “John Citizen” is cited in print since 1919 and “Joe Citizen” is cited since 1935. Middle initials such as ‘Q” and “W” are sometimes used. “Jane Citizen” is cited in print from 1940; this form is usually used in the couple’s name, “Joe/John and Jane Citizen.”
 
Similar names for the average American include Joe Bag of Donuts, Joe Sixpack/Jane Sixpack, Joe Taxpayer/John Taxpayer/Jane Taxpayer, Joe Voter/John Voter/Jane Voter, John Q. Public/Jane Q. Public, Mr. and Mrs. America and Sally Soccer-mom.
 
 
Wikipedia: John Q. Public
John Q. Public (and several similar names; see the Variations section below) is a generic name in the United States to denote a hypothetical member of society deemed a “common man.” He is presumed to represent the randomly selected “man on the street.”
 
Variations
Similar terms include John Q. Citizen and John Q. Taxpayer, or Jane Q. Public, Jane Q. Citizen, and Jane Q. Taxpayer for a woman. The name John Doe is used in a similar manner. For multiple people, Tom, Dick and Harry is often used.

Roughly equivalent, but more pejorative, are the names Joe Six-pack, Joe Blow, and Joe Shmoe, the last of which implies a lower-class citizen (from the Yiddish schmo: simpleton, or possibly Hebrew sh’mo: (what’s)-his-name).
 
(Oxford English Dictionary)
John Citizen, the ordinary man (esp. considered as a member of the community)
1924 H. H. CURRAN (title) *John Citizen’s job.
1931 T. E. GREGORY in W. Rose Outl. Mod. Knowl. 626 If John Citizen buys a house out of his own savings, [etc.].
1937 Evening News 12 Mar. 8/1 No Government department has yet thought of issuing a pamphlet extolling the virtues of John Citizen, who, as taxpayer, every year takes a stronger weight and never complains.
       
Google Books
The Seafarers
By Arthur Corbett-Smith
London: Cassell
1919
Pg. 150:
Not a big fleet action — Mr. John Citizen cannot grasp that — nor a big land battle.
   
13 February 1920, Republic (St. Louis, MO), “Coal Production of Britain May Be Nationalized” by W. L. Mallabar (International News Staff Corresponmdent), pg. 7, col. 1:
Meanwhile “John Citizen” (who represents here the “Mr. Common People” of the American cartoons) is sitting beside his cheerless firegrate with an empty coal bucket and merely wishing that the mine owners and miners would settle their differences and give him sufficient coal to warm him up.
 
Google Books
Liberalism in Action:
A record and a policy
By Elliot Dodds
London: G. Allen & Unwin Ltd.
1922
Pg. 44:
HOW IT HITS JOHN CITIZEN
 
OCLC WorldCat record
John Citizen’s job,
Author: Henry H Curran
Publisher: New York, C. Scribner’s Sons, 1924.
   
16 February 1934, New Orleans (LA) Times-Picayune, pg. 18, cols. 3-4:
There was a time when featherweight felts were entirely imported, and beyond John Q. Citizen’s reach.
 
16 June 1935, Cleveland (OH) Plain Dealer, “Citizen, Don’t Let City See This Dime” by Chads O. Skinner, pg. 14, col. 4:
There was talk early this year at City Hall of calling on the taxpayers for an emergency donation of more than $6,000,000, but—solely because no one connected with the government even imagined that Joe Citizen would kick in that amount—a tax levy for only $5,300,000 was submitted to a vote and carried.
 
19 February 1940, Paris (TX) News, “Presidential Primaries Giver Rank-And-File Voter Chance To Have His Say” by Marogan M. Beatty (AP Feature Service Writer), pg. 1, col. 3:
If the “average voters” do, it will be the first time that John Q. and Mary Jane Citizen, have definitely participated in the Presidential preliminaries on a nation-wide basis in 28 years.
 
12 September 1941, Capital Times (Madison, WI), “Hollywood Gossip” by Jimmie Fidler, pg. 3, col. 1:
From a Hollywood trade paper: “Italian Exhibitors Bidding Big Money For American Movies.” THIS MEANS that Italian fans have turned thumbs down ongoverment sponsored propaganda films and refuse to patronize theaters—and it proves what this column has long contended, that John and Jane Citizen, whether Italian, English or American, will spend money for just one thing—ENTERTAINMENT.
 
3 May 1942, Morning Oregonian (Portland, OR), “Jimmie Fidler in Hollywood,” sec. 3, pg. 2, col. 6:
According to all the recognized authorities, John and Jane Citizen shunned war films, then, for just one reason—they felt too keenly about the great struggle to accept it as a proper frame of entertainment.
 
19 May 1942, The Capital (Annapolis, MD), “Hollywood Sights And Sounds” by Robbin Coons, pg. 4, col. 1:
They are just plain Joe and Jane Citizen who happen to work in a highly publicized and fantastic industry.