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 Norway we have “Ola Nordmann” and “Kari Nordmann”. Ola and Kari are pretty common and generic names. Nordmann literally means Norwegian, but can also be used as a last name.