Answer: She is disappointed that Jing-mei has low expectations.
In this excerpt, we witness a conversation between mother and daughter. The mother is watching a little girl play the piano, but she does not believe she is particularly good. The daughter defends her, by saying that at least she is trying. The mother then uses this opportunity to tell the daughter that she is similarly not trying, but that is she did, she could do great things. This shows that the mother is disappointed at the daughter for having low expectations of herself and not trying hard enough.