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.
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.