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.
The “John Smith” in Chinese is 王小明 (pinyin: wang2 xiao3 ming2), the 王 being the family name and 小明 being the given name. The 王 is a very common family name (like Smith), 小, small, is a diminutive prefix often added to nicknames, and 明, bright, is a commonly used character in given name. I should note that this is only used for males and there isn’t really a “Jane Smith” for females.