Mama is the long-suffering martyr in this story, and the kind of mother and wife who does whatever she needs to for the family.
This means taking a job at a cannery when Papa's sent to Fort Lincoln, working as a dietician in camp for next to nothing, and then taking a cannery job again when they leave camp and Papa can't find work that suits him.
Why does she do all of this? One word—pride. She's willing to shoulder the responsibility of bringing money in because:
Papa would never accept anything like a cannery job. And if he did, Mama's shame would be even greater than his: this would be a sure sign that we had hit rock bottom. So she went to work with as much pride as she could muster. (2.19.14)
But notice that Mama's pride isn't exactly your everyday self-pride—nope, it's pride in the family and her man as a reflection of herself. After all, she made the decision to be with Papa:
In those days Japanese women on the mainland were rare…Most men had to go back to Japan to find a woman, or take their chances on a 'picture bride.' Mama was worth a lot, and before she finished high school they had promised her to the upright son of a well-to-do farmer in the territory. (1.6.13)
In other words, since she's the one who passed on the good guy to hook up with the bad guy, if he and the family go down, then that means she and the choices she made in her past go down too. Which is also why, when they go to Jeanne's academic awards ceremony at her high school, Mama "smile[s] continually, smile[s] at everyone, as if to make up for Papa's solemn courtesies" (2.20.30). That's what Mama does—she makes up for Papa.
It's also what every Japanese American wife seems to do:
Early each morning [Mama] would make up her face. She would fix her hair, cover it with a flimsy net, put on a clean white cannery worker's dress, and stick a brightly colored handkerchief in the lapel pocket. The car pool horn would honk, and she would rush out to join four other Japanese women who had fixed their hair that morning, applied the vanishing cream, and sported freshly ironed hankies. (2.19.14)
Family first… even if mostly as appearance sometimes.