I think that it makes a difference whether there is any chance that the person using the pseudonym could be confused with a more well-known person with that name.
There are a number of users who have chosen the pseudonym "Gauss" (or something similar). Obviously, nobody is going to think that Carl Friedrich Gauss has come back from the grave to post questions on MO. I don't think that such pseudonyms are any worse than a random pseudonym.
Conversely, intentional attempts at impersonation or identity theft are obviously not okay. Even non-malicious or thoughtless use of pseudonyms that could cause confusion are IMO a bad idea and should be discouraged if not banned outright.
"Alexander Grothendieck" is a borderline case. The famous Grothendieck is no longer alive, although some people might not be aware of that because he died only five years ago. Even if there were no risk that anyone would think that the user "Alexander Grothendieck" was the Fields medalist, the use of his name might be interpreted as a disrespectful gesture. I would not recommend banning the use of such a pseudonym, but perhaps the moderators could approach the user in question and gently suggest that a different pseudonym be used. Chances are the user thought that the use of the pseudonym was either harmless or even a sign of respect, and I think that a heavy-handed over-reaction would be counter-productive.