Muns are the RL people/RL LJs behind the characters. Ethymology unknown. To someone who knows me only fleetingly from the discworld_rpg, I am Tantony-mun, or Margolotta-mun, or whichever of my characters they're referring to at the moment.

Some of it might be snobbery, yes, especially if you're some sort of minor star in fandom or think they would like to be. But I don't think that's why the people on myf-list are doing it.