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.
Japan, you usually use “Taro Tanaka” as the goto placeholder name. Tanaka is a very common surname here and it’s super easy to write as well (田中). Sato is actually the most common but also a lot harder to write (佐藤). Tanaka is also a very “working class” name, it litearlly means “in the rice field” and most likely comes from families with a background of rice farmers. Taro also a very common given name; there have been quite a few Taro Tanakas throughout history despite being the placeholder name on most forms and the like. Taro, incidentally means “Big/Strong Son”.
I am not sure offhand of a female equilvalent, it’s usually Taro as the placeholder.
Along with Tanaka, Yamada (山田) is also often used as a placeholder surname. It’s a comparably common name that’s also simple to read and write.
As for female placeholder given names, IMO the closest equivalent to Taro would probably be Hanako (花子). I’ve seen adverts that show sample credit cards or forms bearing these names.
For example, an ad for a Toyota financing promotion might feature the name Toyota Taro on the sample application form. Or in a Mitsubishi UFJ credit card ad, the happy-looking lady in the ad will be using a credit card that bears the name Mitsubishi Hanako.