Actually, in LARP specifically, I think it's entirely appropriate to have tonal variation. That's because a LARP typically isn't *one* story, it's a whole *bunch* of stories, each with a different protagonist and plots. It's common and reasonable for those stories to be tonally quite different, so long as each is *internally* consistent.

So for example, in Tabula Rasa II, we had both the psycho serial killer (whose character sheet was the darkest and creepiest thing I've ever written -- half of it is an argument with the voices in his head) and the gentle wacko who basically thinks he's Batman. Each story works well when viewed from its own perspective, but looking at the character sheets side-by-side would be bizarre...