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 the Netherlands, it’s either “Jan Smit” (both a very common first and last name, but also a local celebrity’s name) or “Henk de Vries” which IIRC is the most common name here.
I also vaguely remember some old Dutch forum naming everyone Henk de Vries by default.