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.

The significance of the scene in which Mama breaks the dishes is that it shows that she is a proud woman who is not willing to be taken advantage of.  She is determined to maintain her sense of dignity and to, as much as possible, avoid being used by the people who are abusing her and her family.

Mama breaks the dishes when the government has given the family a mere 48 hours to move farther away from a particular naval installation.  She cannot take the dishes with her because there is no room in the car.  A second-hand dealer offers her only $15 for dishes that are worth $200.  Instead of letting him cheat her like that, she destroys the dishes.

By doing this, Mama is showing that she has her pride and her dignity.  She would rather get no money while keeping her pride than allow this man to take advantage of her.