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.
I think this one is pretty confined to my region (southwestern USA) but we use Otto as the moniker of a generic stupid person too, but probably for a different reason: Otto is Oblivious to the Obvious