I'm noticing they do this with a lot of trailers for animated movies these days regardless of the movie's content. Do a trailer that'll make the kids giggle and then do the movie whatever way you want.I agree. I read Peanuts growing up, and I've read much of the collected reprints, including some of the really old strips when Snoopy was just a puppy. Peanuts has gone through a lot of shifts in tone over the years, and the only thing really out of character in this trailer is the soundtrack, and I don't have a problem with that. It's a little strange for pop music to be associated with Peanuts, but I think it works for the trailer (and I can't find any evidence it will be in the film).
Tangled is probably the most notorious, with the whole hair attack scene ... none of which was in the movie. I passed up on that and Megamind because the trailers made the movies come off as really stupid, but then later found out they were really good movies that the trailers had misrepresented.