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 Philippines, it’s Juan and Maria dela Cruz, although those have fallen out of use due to the popularity of Western (aka US) culture. Interesting reading about every country’s own names for their everyman.