I think it's about her being classy, inteligent, successful and totally hot. But they mimic her accent and make semi-racist comments(saying that Indians are skinny because they all cook chicken the way Shilpa does), so it's not surprising that people come to the conclussion that it's race related. And Jack did call her a paki. Which is inaccurate as well as clearly racist.
Either way, they're picking on Shilpa constantly, for anything and everything. They deserve all they're gonna get.