It depends, probably Max Müller or Hans Müller, just anything Müller. Unlike the US, we don’t normally use a real name as placeholder name [0], the options there would be Max or Erika Mustermann (literally example or pattern man), for an average person it’s Otto Normalverbraucher (Otto Average-Consumer) and Lieschen Müller.

[0]: https://en.wikipedia.org/wiki/Placeholder_name

I thought it was John/Jane Doe but I now realize that those are only for unidentified people (in police/judiciary contexts).

In Italy - for the record - it would probably be Mario Rossi (as an example on a mockup form), but conversionally it would be Pinco Pallino.

There are a few dedicated English wikipedia pages:

https://en.wikipedia.org/wiki/Placeholder_name

https://en.wikipedia.org/wiki/List_of_terms_referring_to_an_...

https://en.wikipedia.org/wiki/List_of_placeholder_names_by_l...

it is interesting to see the slighty different use in the various languages.

There was a John Smith who was briefly the leader of the Labour Party in the UK (he sadly died very soon after becoming leader from a heart attack), and I remember him saying that before he became well-known, when he checked into hotels with his real name the receptionist's often eyed him suspiciously, thinking it was a false name to cover up the fact he was checking in with his mistress.

Peter Müller (i.e. Peter Miller) would be a good generic one.