My headcanon is that other kerbal surnames do indeed exist, but the KSC is run almost exclusively by Kermans. The Kermans are actually all clones sharing a common trait: an obsession with the pursuit of space. That is why the space tourists all have that surname too. Non-Kermans, especially by comparison, tend to be afraid of or disinterested in space, preferring to focus on their Kerbin lives instead. The area of the KSC and its nearby towns is rather surreal to outsiders, given that the majority of its population consists of these clones.
But as far as game canon goes, I do find it peculiar that all the characters share the Kerman surname. If the first names can be randomly generated, then why are the surnames not? And even the non-generated kerbals (Such as Gene and Wernher) have that surname and it's never really mentioned why. It's strange but I like it.