The main reason that Jordan Baker is described as begin contemptuous is because she is a scornful and disdainful character throughout the book. She, like Tom, looks down on others and does not really care much about anyone but herself. We can tell through the conversations that she has with Nick that she really does not have anything positive to say about anyone. Throughout the novel she is characterized as someone who sneers at anything that does not have to do with her or something that benefits her.
Jordan Baker is one of the more complex characters in this novel, and it's not clear that Nick ever fully understands her. I would argue that she uses her scorn as a defensive mechanism to keep people and their messy emotions at bay. For instance, when Nick attends one of Gatsby's parties for the first time, he starts to get "roaring drunk," then runs into Jordan. She looks "with contemptuous interest down into the garden" as a way to distance herself from Nick's "loud" behavior, though she does let him join her. It's her compassion in taking the drunk Nick on at the party that we must note, as we note that she helps Daisy pull herself together when she gets drunk on her wedding night.
But while Jordan helps people, she also tries to distance herself from their messy emotions. When Gatsby and Tom are quarreling over Daisy in the Plaza, Jordan first tries, with Nick, to leave, then withdraws emotionally, seemingly balancing "an invisible but absorbing object on the tip of her chin."
At one point, Nick attempts to analyze her and says her "bored, haughty face ... concealed something." He sees her as an expert in "subterfuges" who began while young to keep her "cool, insolent smile turned to the world and yet satisfy the demands of her hard, jaunty body."
Nick may misread the reasons for her concealment, but he does perceive that she projects a protective facade and that a contemptuous look is part of this. That Jordan is not simply hard also comes out at the end of the novel, when she runs into Nick and tells him that although she had thought he was careful and honest, he's not. She cares that he broke up with her over the phone, a cold thing to do. Despite her facade, it's clear he disappointed her.