It is no more true that Obama is unpopular because people call him a "socialist" than it was true that Bush was unpopular because people called him a "fascist." You are confusing cause and effect.

I think foreign observers think that the American public is atavistically motivated by invocations of "socialism" because they view the US through the lens of their own politics. In reality, Obama is unpopular because the economy is in the shitter and unemployment is running near 10%. It doesn't matter whether that's fair or not.