It was like she was mad all the time. [...] Only there was nothing to be mad at. Unless the store. But the store hadn't asked her to take the job. So there was nothing to be mad at. It was like she was cheated. Only nobody had cheated her. (3.3.15)
Mick's life is a long series of disappointments, and her job at the store is most disappointing of all. That job can never fulfill a musicmaker like Mick, but she takes it to lift herself and her family out of poverty, which has been a source of frustration for them for the whole novel. Here, she is caught between satisfying her need for money and satisfying her need for fulfillment.