He did? or was it one of the 50+ writers they employed?
Most likely one of the writers.
My point was the Letterman was entertaining for the first five or so years he was on TV. Then after losing the Tonight Show to Leno, he turned into an A$$H*Le.
He was never a stand up comic prior to his show. He was a TV weatherman from Indy. He does not have that natural talent/timing in telling a joke. He was funny when he played the fool. Then he stopped playing the fool and he was not funny anymore. He was just bitter.