It’s a regional/religious difference. In the southern more Catholic regions it’s mostly Schmitt and in the northern more Protestant regions it’s mostly Schmidt.

I think just similar names.

In Austria Maier is very common, but also Meier, or Mayer.

‘Mustermann’ is more like an artificial placeholder name, that gets used on facsimilies of passports and drivers licenses used as example illustrations.

“Muster” in that context also means something that is only for demonstration purposes, not the real deal. That word is also printed across prints of Euro-bills when they are depicted somewhere in order to avoid charges for producing counterfeit money.

Afaik there are actual people with that last name, but that’s pretty rare.

I was thinking Mustermann is more like John Doe in that regard, but John Doe is also used for a hypothetical regular, average person and we have “Otto Normalverbraucher” for that use-case. (“Normalverbraucher” literally means ‘normal consumer’, no real person has a name like that)

OP’s question is aimed more at a last name, that is very common and stereotypical, almost boring. While the close translation of Smith Schmitt/Schmidt/Schmid also fulfills that criteria the even more regular one would be Müller and Mayer (or one of its spelling varieties)

Those three names are so common that “Müller-Mayer-Schmidt” has become another phrase used to refer to the average citizen archetype.