As long as the person is identifiable and there is no doubt as to who you meant, this should not affect the validity of the choice/appointment of them as executor. It would typically only be an issue if:1) After you pass, the lawyers, court, etc. involved can't figure out who the executor is; or 2)  There are two people who, based on the nickname, it could be, and disagreement as to which one it is. (E.g. you wrote that the executor is "Curly Franks," because you call your bald friend Ken Franks "Curly"--but unfortunately, you also know another Franks, Tim Franks, who went bald and was also sometimes called "Curly," and both Ken and Tim think they were named exectutor.)If you have any doubt that after you pass, they will be able to find and identify the executor, then redo the will or add a codicil to it making clear who the executor is.