Think: Kramden and Norton- dumb Lucy and Ethel - dumb Riley and Gillis - dumb Amos n Andy - dumb Toody and Muldoon - dumb
Do you detect a pattern? Dumb = funny. (Actually, Amos wasn't so dumb, but neither was he funny.) Aside from Bilko, who was a crook, can you think of a smart character in a 50s sitcom?
For the most part, I agree with your premise -- that people in a movie or TV sitcom typically act in a stupid manner in order to be funny. Laurel and Hardy are the probably the best examples.
However, I question whether Lucy Ricardo was "dumb" in any sense. She was always conniving, always coming up with schemes to get her way. Sometimes she was successful, sometimes not, but her brain was always active and always productive of new ideas. I'd say she was pretty smart -- like the Ward Cleaver mentioned in another post here, if you will -- and yet she was undeniably funny. Is Lucy Ricardo the exception that proves the rule? Maybe so. Lucy broke the rules all the time!
In any case, I can think of other examples of "smart" characters being funny. How about any one of the Marx Brothers? And was Ethel Mertz really "dumb"? In the "I Love Lucy" series she arguably came across as neither smart nor dumb, but she too was able to produce humor. For stand-alone comedians, how about Sid Caesar? His brand of humor was highly intellectual, but it was always very funny.
I suspect that the "smart" characters who fail to be funny are simply those who are assigned the role of being "wise." (Not wise-ASS, like Groucho, but wise!) Ward Cleaver, Jim Anderson (played by Robert Young) and Amos Jones were all supposed to be wise and all-knowing, so naturally their focus was not on eliciting laughs.
I can't offer an opinion about Danny Thomas because I don't think I ever saw his show. As to Lucy, I haven't seen much of her show, but what I remember is dumb stuff, like her getting drunk on megavitamins, or stuffing chocolates down her blouse when the assembly line speeded up. (Fenwick Babbitt did the same routine but with pies.)
Sid Caesar was a law unto himself, but his show was not a sitcom. His character in the recurring Hickenloopers sketches was, arguably, not very bright.
But the few exceptions to the rule prove the rule that, to sitcom producers, dumb = funny, and my point is that Amos n Andy should be considered in that context.
After 'I Love Lucy' , the requirement was to have a dumb man (father) and smart woman (mother). This formula became the basic skeleton for almost all sit-coms after. Still being used now. Just try to make a list of smart men, dumb women shows and compare to the number of smart woman, dumb man shows.
Smart men, dumb women: I MARRIED JOAN MY LITTLE MARGIE (though "Dad" was often caught in Margie's schemes) MY FAVORITE HUSBAND (Lucille Ball starred in the radio version; CBS wanted her to transfer it to TV but she refused, wanting to have a vehicle that could co-star her husband, you-know-who) PETE AND GLADYS LIFE WITH ELIZABETH Smart women, dumb men: BLONDIE (Barbara Britton/Arthur Lake version) THE LIFE OF RILEY THE REAL MCCOYS
"We're fighting for this woman's honor, which is more than she ever did."
The point I tried to make in 2009 was that dumb characters are standard for successful sitcoms, so the fact that Andy and the Kingfish were dumb was not a racial slur but a sitcom requirement. I've never heard anyone claim that Gleason and Carney's Honeymooner characters were insults to the Irish.
Once again, the criticism of A&A came mainly because it was virtually the ONLY depiction of blacks on TV at the time; there was no alternative. No black doctors, no cowboys, no private eyes, no nurses, no black kiddie-show hosts, no soldiers, no soap opera heroines, nada. There were all kinds of alternatives to the imbecilic Kramden/Norton tandem. May I bone your kipper, Mademoiselle?