Letterman was funny early on much like George Carlin, but both slipped into, what? A combination of mediocrity and bitterness. I saw Carlin in person later on and he was almost unintelligible. He was reading off of cue cards he held in his hand and seemed unconnected to not only his audience but to where he was. They are two peas in a pod to me.
Letterman was funny early on only when he was using comedy bits stolen from Steve Allen during the first Tonight Show back in the 1950s.