When Mama breaks the dishes, it shows that she won't be a victim, even in the most trying of circumstances. Her behavior is also significant, because it highlights the suffering endured by many Japanese Americans during the war.
Wakatsuki's family faced a dilemma experienced by many Japanese Americans during the war. They didn't relate to the hegemonic ambitions of the Japanese military. However, they also didn't fit into American society once the war started—or rather, American society refused to let them fit in. Many Americans viewed their Japanese neighbors with suspicion, as many believed that they were secretly spying for Japan. While there were some instances that justified this fear—such as in the case of Takeo Yoshikawa, who played a part in the Pearl Harbor attack (see link below)—the majority of these Japanese Americans were ordinary citizens.
Thus, Wakatsuki's family experienced rejection at almost every turn. In the incident above, Mama breaks the expensive China dishes because of her deep frustration. However, her actions also highlight her proud and indefatigable spirit. She isn't about to be defeated by life's circumstances, nor will she submit to the machinations of second-hand dealers.