C3P0 = being too smart and too sheltered for your own good is funny.
Jar Jar = being too stupid and too outgoing for your own good is funny.
At least that's what I think the idea was behind the two characters. Out of the two characters I can safely say that I find it a lot easier to laugh at the smart person going through misfortune than the stupid one...
On the other hand Artoo was the forgotten hero of the movies who saved the other heroes lives time after time but was never properly thanked by any of them.