Mama's pretty much a combination of the nicest qualities that a kid can remember his mom having; she's quiet and unassuming, yet very much present. The first time we meet her is when Hassan talks about when his parents met, and Papa fell in love with her "intoxicating whiff of chapatis and rose water" (1.26). Comforting, right? Mama is shy and smart, and controls her loud and sometimes blundering husband with a steady hand. She also takes good care of her children.
In Chapter 2 Hassan talks about his outings with her. Mama loves to shop for scarves and takes Hassan on secret trips to get a break from the chaos of home, and on these outings, she gives Hassan his first taste of French food. She also tells him to "'never be afraid of trying something new […] it's the spice of life'" (2.61). In other words, Mama totally helps plant the seeds of Hassan's career.
Even though she dies in Chapter 2, Mama is mentioned as an anchor of peace throughout the rest of the story, always reminding the family of the right thing to do. So when Mallory protests for days outside the Maison Mumbai and Papa refuses to cease fire, Zainab tells Papa, "'Mummy would want us to stop running'" (11.140), and almost instantly, Papa gives in and allows Hassan to go work for Mallory, which turns out to be a hugely important decision.
She might be quieter and generally more demure, but just like Papa, Mama's presence lingers on long after she's gone.