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.
In Poland we have “Jan/Anna Nowak”, the most common first and last name.
Not entirely relevant to your question, but funny/interesting: boomer memes were quite popular a while ago, and from those we’ve got "Janusz " and “Grażyna”, typical boomer first names. There is for example “Janusz biznesu”, meaning “Janusz of business”, a person that shamelessly uses malicious or shady business practices.
Cliff Tom Robert, David most male boomer names are still somewhat popular except maybe Norman. Not a lot of Normans.