I'd disagree completely :3 I think Ghost types are really just as diverse with their personalities as any other type of Pokémon. Sure there's a couple like Duskull & Drifloon that'd probably be all cold and quiet and as you said yourself there's several mischievous Ghost types like Gengar and Rotom, but I reckon the majority of Ghost types would have very different personalities. Golett strikes me as a really jolly and friendly Pokémon that'd plod around interacting with anything it saw, Cofigrigus seems to be a really aggressive and protective Pokémon, Giratina seems like the powerful overlord-y type, Frosslass seems like the type to wander around aimlessly for days just exploring the land, Frillish seems like the type to help out anything that passes with a really cheerful manner, etc etc, basically I just think they're really diverse was the whole point. xD I might be wrong though!
I'd like to level Rotom c: