Groucho was originally invited to play the part that was later changed to be Sylvester's mother (he would be the father). It would be absolutely hilarious, but unfortunally he had scheduling problems and had to decline the role.

It's not "unfunny" in the "Friends" kinda way (absolutely unlaughable), it's just that the jokes are sometimes "over the top" and too slow paced, leading to nowhere (Sid Ceaser and the fireworks comes to mind). The movie is dated and too long, and the jokes are tossed in between the long "dialogue and driving" scenes. How funny is it to see 5 times the same Sylvester scene, yelling "I'm comming Momma!" with a road being projected behind him? And couldn't they have used the three stooges and Jerry Lewis a little bit more? I mean the stooges were absolutely out of job at that period, they could have been in the final climax scene... And the jokes are not witty and smart like, let's say, Pink Panther (which was produced around the same time).

Besides I always felt bad about Spencer Tracy's character, so it kinda distracted me a little from the comedy.