OED attests 'corn' in the 'corny' sense with quotes from 1936, 1937, and 1946. The third meaning of 'corn' that may have been bubbling toward the surface of Hammerstein's compository stew was the elliptical
YourTango: 30 Cheesy Love Quotes That Will Make You Laugh (Even If They’re Corny)
30 Cheesy Love Quotes That Will Make You Laugh (Even If They’re Corny)
Why do we call dull, old-fashioned or banal things corny? As in corny movie scenes or corny jokes; not, vegetable or corn related characteristics. This blog article I found on Google suggests it
(Please note - indeed @Dan Bron came up with the word exposition, which is now used to further clarify the question.) There are many specific terms for different types of corny exposition in films....
The OP's explanation indicates that it includes bad telling of the joke messing up the joke, which doesn't really fit under "corny joke".
2 "cornball" NOUN: a person who tells "corny" jokes, a rube or bumpkin "When it comes to telling jokes, he's a real cornball." Oxford Dictionaries Online "corny" ADJECTIVE: trite, banal, mawkishly sentimental "His jokes are so corny."
The OP states confidently in their question Why is “folks” commonly used as a gender-neutral term for “people” when “people” is already gender-neutral? The term folks, originally an Old English term used on both sides of the Atlantic, has not seen a recent surge in popularity because it is a better alternative to the gender neutral people, but because it sounds friendlier, and more ...