The last lines of "Harrison Bergeron" are deliberately depressing, showing that there is no hope for the future society. Instead of breaking free of their handicaps and overthrowing the corrupt government structure, the populace is doomed by their own inaction to live their lives in stagnation, without any new ideas, great deeds, or real change. Hazel, who is described as having "perfectly average intelligence," is a perfect citizen, unable to think about things "except in short bursts." Although she and George watch Harrison's extraordinary performance on television, and see him killed without mercy by the Handicapper General, they are prevented from remembering it: she by her own intellect, and he by his handicap.
"That's my girl," said George. He winced. There was the sound of a rivetting gun in his head.
"Gee -- I could tell that one was a doozy," said Hazel.
"You can say that again," said George.
"Gee--" said Hazel, "I could tell that one was a doozy."
(Vonnegut, "Harrison Bergeron," tnellen.com)
The common phrase "You can say that again" is usually meant in a sarcastic way, to denote that a comment was overly-obvious. When Hazel says that George's mental handicap noise was "a doozy," she is not being sarcastic, but simply making conversation. As she has average intelligence for this future world, she misunderstands George's comment and repeats her own. This shows that, without some major world event, the society will continue to stagnate and eventually collapse as its inhabitants lose all semblance of rational thought.