Explanations/etymology also appreciated!
For Joe Shmoe, it means a very average or below average person. It’s a derivation of the practice of using “shm-” to dismiss something (eg “Practice shmactice. We’re already perfect”).
And “John Smith” is meant to be the most average name or person imaginable, so they have the “most common” (citation needed) first and last name as well.
Interesting. In the UK (at least when I was growing up, I haven’t lived there for some time) we called doing up shit cars as Barry-ing them. I know in other parts of the country they used the name Ned or Kev to refer to the people that drove those cars.