@miriam98 - Graffiti is used for nostalgia? I don’t think so. In my opinion, It doesn’t matter where the name came from. You could parse it out and say it was a mangling of the word “kill joy.”
That would lead people to believe that the Kilroy was some guy who was bent on raining on your parade. That’s as good a theory as any, but like I said, it makes no difference.
Every culture has come up with its own variant of this omnipresent character, and it’s more important to understand its usage rather than knowing where it came from.