The dumbest movie that I have ever seen would have to be Jingle All The Way. It had a rather stupid plot of father doesn't show up for boy's karate show tihng, boy then begs him for a Turboman doll, man discovers mass hilarity having to do with Turboman at toy store, Turboman takes over man's life, boy begs man more, eventually man ends up getting dressed up as Turboman and being in a parade. Stupid. (The kids in my general music class last year voted for this, so that is how I ended up watching it.)
The second-most stupid movie would have to be Home Alone 3 (again winning a general-music voting last year). I do not overly well remember the plot of it, but I do remember that it had to do with putting some secret thing inside a radio-controlled car or something, a boy ending up with it, and the boy using it to drive around a video camera to look for a burglar. Lousy, and both of these movies were enjoyed by those kids.
I typically do not watch stupid shows, but I saw commercials for MTV2 a while back. The commercial advertised "Stankervision", "Damage Control", and "The Andy Milonakis Show" and showed people running(?) on swimming pools, pulling down underwear slightly to put a "d**n" sticker on it (It did not actually say "d**n", but I think you would know what I mean.), and smashing car windows. What a waste of film!