In ancient Greece, the people of Kyme (and Abdera) were proverbially stupid. Here are some jokes about them from the ancient collection known as the Philogelos or 'Laughter-Lover'.
A man from Kyme was trying to sell some honey. When someone came and tasted it and said that it was very good, the seller said: "Well, yes: if a mouse hadn't fallen in it, I wouldn't be selling it!" (173)
When a distinguished man was being buried in Kyme, someone came up and asked the mourners: "Who was the dead man?" One of the Kymeans turned around and pointed and said: "That guy lying on the bier." (154)
A Kymean doctor, operating on someone who was in terrible pain and crying out, switched to a blunter scalpel. (177)
Here are a couple of non-ethnic jokes from the same collection (I quoted another on March 5th):
Posted by Dr. Weevil at July 28, 2002 11:12 PM
A professor on a sea-voyage, when there was a big storm and his slaves were weeping, said: "Don't cry. I've set you all free in my will." (25)
Returning home from a trip abroad, someone visited an incompetent prophet and asked him about his household, and he said: "They are all healthy, including your father." And when the man said, "But it's been ten years since my father died", the prophet answered "You don't know your true father." (201)