How could "Harrison Bergeron" be interpreted to show that social Darwinism is good?
Since Kurt Vonnegut is deliberately satirizing the concept of forced social equality, the argument that social Darwinism (survival of the fittest in social matters such as economics) can be shown as good in the story must take a very specific view.
If one assumes that the eventual equality of result is the goal, rather than the equality of opportunity, the world in "Harrison Bergeron" is very successful; by legislating handicaps on capable people to bring everyone to the same basic level, all people have the same opportunities and ability. There is no personal exceptionalism, and no one is left out or made to feel inferior.
The year was 2081, and everybody was finally equal. They weren't only equal before God and the law. They were equal every which way. Nobody was smarter than anybody else. Nobody was better looking than anybody else. Nobody was stronger or quicker than anybody else. All this equality was due to the 211th, 212th, and 213th Amendments to the Constitution, and to the unceasing vigilance of agents of the United States Handicapper General.
"If I tried to get away with it," said George, "then other people'd get away with it-and pretty soon we'd be right back to the dark ages again, with everybody competing against everybody else. You wouldn't like that, would you?"
"I'd hate it," said Hazel.
"There you are," said George. The minute people start cheating on laws, what do you think happens to society?"
(Vonnegut, "Harrison Bergeron," tnellen.com)
Taking the assumed goal of equality of result, this future world is extremely successful; since everyone is equal, mentally as well as physically, they cannot form the individualized thoughts that would temper unhappiness. If everyone is happy, and society runs without failing, then the social Darwinism must be a good thing. In this mode of thinking, Harrison himself is evil, because he seeks to exercise his personal ability, and prove that he is both able and exceptional. If he is allowed to be superior, that means that others must be inferior, and that destroys the idea that equality of result can be achieved.