As far as I'm aware (as an American) she's fairly popular here. Nowhere near the level of the Williams sisters (obviously) but she got a lot of attention last year, particularly from the US Open (be that good or bad). That said, there certainly isn't an animosity towards here that i've noticed. She would probably be more warmly embraced if she represented the US rather than Japan, but that never held Sharapova back....
Could also be that her personality, while charming, is a bit quiet and reserved. Us Americans like our loud, flashy characters a la Serena, Roddick, Agassi, etc.