I generally hate "dumb" humor as well. There are enough dumb people I encounter throughout the course of my life, I don't need to expend time and money to see them on the screen. The difference with a movie like Airplane! is that they aren't trying to be dumb. They are playing it straight. That's what made it so hilarious, to me anyway.
Chad blew the call, let's not forget that. HAHAHAHA.
Anyway, Chris, you nailed it. Airplane, and to a lesser extent, Naked Gun, played it straight and that's the magic. When Leslie Neilson is searching the desk, opens the drawer, and goes "BINGO!" then pulls out the bingo card, I laugh every time. Jim Carrey, rarely makes me laugh. I just don't go for that goofy, silly, "I'm CRAZEEE!" type of humor.
That's why, to me, Beavis and Butthead are funny. I KNOW people like that. They're dead serious, in their own way.