One of the most important motifs in this short story is that of the American Dream and the power it exerts throughout the story, on both Jing-Mei and her mother, but also on the other characters mentioned (for example Waverley and the other Chinese families that Jing-Mei's mother brags to).
The American Dream is expressed most succinctly by Jing-Mei's mother who says "you can be anything you wanted to be in America." Coming to the land of unfettered possibilities gives so many more opportunities to immigrant families - far more than we know Jing-Mei's mother had in China. Thus we can understand the power this has on immigrants, and in particular the pressure there is for second-generation immigrants (immigrants who have been born in the States) to make the most of these opportunities. Thus we can perhaps understand the pressure that Jing-Mei is under.
The problem that Jing-Mei's mother does not forsee that although there are unlimited choices in the States, those choices also include the choice to not excel and the choice to be normal, which is what Jing-Mei chooses to do. Coming to a land of freedom necessarily involves more freedom than perhaps we would expect.