In the US, "Joe the Plumber" isn't a substitute for "Joe Six-Pack" or "Joe Smith," its a specific reference to a famous controversial person. That's like me calling the average Italian a "little Mussolini."
It was a general reference referring to no one in particular, before the election of 2008.
No, that was still Joe Sixpack and Average Joe and Joe from Main Street. Joe the Plumber was always a specific person, but it didn't matter while he was a proxy.