Here's a nice descriptive touch about Mama: she smells like "sunshine and soap" (6.106).
Okay, need more of a reason to care, other than that she's clean and hopeful? Mama is the mother (duh) of Cassie and her brothers. She's intelligent, educated, and caring—and she's also a bit of a "disrupting maverick" who doesn't believe the status quo should be accepted (1.174). Cassie really looks up to Mama. Check out this quiet moment, when Mama fixes Cassie a "grown-up hairdo" for church (6.105):
Then she wound each braid into a flat chignon against my head. My hair was too thick and long for me to do it well myself, but Mama could do it perfectly. I figured I looked my very best that way.
"One day, Mama, you gonna fix my hair like yours?" (6.109-11)
So, why does Cassie admire her mother so much?
Well, here's a reason we admire her: she teaches her students "radical" (1.174) material about slavery that isn't covered in the county-approved textbooks. Cassie spies on Mr. Granger while some of the school board members observe Mama's class. Even though the white men are watching, she talks about slavery, and "she spoke on the cruelty of it; of the rich economic cycle it generated as slaves produced the raw products for the factories of the North and Europe; how the country profited and grew from the free labor of a people still not free" (8.101).
And she doesn't back down. Mr. Granger tells her that she shouldn't teach what's not in the book, but Mama insists that "all that's in that book isn't true" (8.105, 8.107).
So that's where Cassie gets her sassiness and strength. Mama isn't afraid to stand up for what she believes in and what's right. Even if it gets her in trouble. And it does: Mr. Granger has her fired.
Mama also organizes the boycott against the Wallace store. She's tired of complaining and says so:
Everybody from Smellings Creek to Strawberry knows it was them but what do we do about it? We line their pockets with our few pennies and send our children up to their store to learn things they've got no business learning. [...] As I see it the least we can do is to stop shopping there. It may not be real justice, but it'll hurt them and we'll have done something. (7.58)
In other words, Mama is one strong woman who is not to be trifled with. The community trusts her and initially goes along with the plan. She's a natural-born teacher and leader, and we get the feeling that Cassie will grow up to be similar. At least, we hope so.