What I find interesting is that in the 1970's the two major comic autuers (if i'm wrong about this and missing someone, feel free to correct me) were Mel Brooks and Woody Allen. People who put "Spaceballs" on a list of best stupid comedies would be surprised to know that Mel Brooks was actually considered a very respectable filmmaker by the critical community. Of his first 4 films, 3 (The Producers, Young Frankenstein and Blazing Saddles) are on the AFI's list of top 100 comedies of all time, and all of them ranking in the top 13.
His first film "The Producers" (1968) which was a humongous critical and commercial success on Broadway this past decade, put him on the scene and his next big hits were both made in the same year, 1974: "Young Frankenstein" and "Blazing Saddles."
I researched Young Frankenstein's critical reception for a project in a film class a couple of years ago and that got great reviews from the New York Times, LA Times, and Long Island's Newsday. Frankenstein and Producers were comically intelligent films but didn't really push the envelope of the border between high-brow and low-brow entertainment that much. Young Frankenstein established Brooks as a master of parody: the genre of choice for pretty much the rest of his career. But Young Frankenstein was not really very low-brow at all. It had no bathroom humor and had sexual situations but used those in a way that evokes the innocent mix-ups of a screwball comedy. It also didn't go for cheap one-liners. Its subtlety was in its attention to detail in mimicking the original movies and perversing it ever so slightly.
Blazing Saddles was a different story. It was still relatively intelligent with its humor and a film academic could appreciate its clever metacinematic ending in which a brawl in a Western town spills over onto a studio lot and a movie villain tries to escape his own fate by going into a movie theater. It was also just plain hillarious and jam-packed with laughs that left audiences and critics defenseless. At the same time, the movie repeatedly dipped into crude humor and had the potential to be offensive in the way that his later films would soon follow the pattern of. It's possible for a movie to both deliver intelligent humor and be really stupid at the same time and I would give a good review to Blazing Saddles for its intelligent humor but I would be doing so because its pros outweigh the cons.
I didn't neccessarily like the dumb humor and that's where his movies have increasingly followed to. I have a theory that because of the decline of critical respect for Brook's films and because he was one of the leading comic filmmakers of his day, he played a large role in what makes comedy less respectable today.