This is such a great thread, well done Chickenfeet! I think there are loads of really valid points. I think there are levels within everything, and comedy is perhaps more unforgiving as a medium; I think those who do satire well - Tom Lehrer, Flanders and Swann, no one's mentioned Randy Newman or Jake Thackray yet! - that stuff bears repeat listening. Great writers like John Prine, Leonard Cohen, Bob Dylan, Queen, often drop humour in but are not really 'comedy'.
But maybe comedy songs are not always best represented by recordings, but rather by live performance, so they don't have to be listened to over and over again. So often songs have turned up for one off performances on shows like TW3 or That's Life (or the Muppets), the equivalent of seeing Marie Lloyd in the music hall. Victoria Wood probably didn't sell many albums, I'm guessing? But people know her and love her for the stuff that they may have only been exposed to once or twice (apart from the immortal Freda and Barry, of course)! And I guess the best comedy songs do more than just make us laugh - possibly Benny Hill achieved this with the hard-done-by hero Ernie!
PS great first response by Skub!
PPS I think my next posting will be a children's song! Ay caramba!