I think the clothes are meant to highlight how anachronistic the Stupid family are. The world of the movie is real, except for the Stupids themselves (their dog is stop-motion). The annoying part is their behaviour. I mean, they are supposed to be stupid, but they are annoying in that stupidity, not amusing, if you see what I mean...
I think they were intended as a kind of family of Clouseaus. It's like a Mr. Bean movie with four Mr. Beans. It's just too much. 16 years on and it's still annoying.
Maybe it doesn't help that I know someone like that in real life. His mother drops us off at the station. Train is on the platform ready to leave, he wants to run back to the car to get his sunglasses.
Another time, he managed to elbow a girl in the face while chatting her up. His default method of attracting the opposite sex is "WOOOOOOOO!" (I think that's what he thinks 'wooing' means). He also considers himself a hit with the ladies when all he does is pester them into submission.