Mama seems like a loving, very serious person who is intensely distressed by Doodle's condition. Mother's instinct is to be completely protective of her fragile son. Yet, interestingly, she encourages Brother and Doodle to spend their time together. On the one hand this is awesome, because otherwise Doodle would have been isolated. He might never have learned to walk, nor have been able to build a relationship with Brother.
Yet, if she had been more aware of what Brother was up to, just a bit more observant, she might have put a stop to things before they got out of hand. We don't say this to cast blame on her, but rather to show that Brother is not solely responsible for what happened. Adults were around who might have averted the tragedy.
It looks like Mama did a big turn-around during the time period in which the story takes place. Initially she can't imagine Doodle being an independent person, and is overly protective of him. But, as her son becomes more and more "normal," she forgets that he needs her extra protection. She tells Doodle, "Don't you dare touch [the dead ibis]. […] There's no telling what disease he might have had" (6.35). This seem protective, but she's treating him the same way she would Brother in the same situation when he was six.
She isn't on the look out for unusual problems, like his extreme exhaustion, which she probably thinks is normal and not a big deal. On the one hand, this is positive because it means she completely accepted Doodle as a "regular" boy. Why it's not so good is obvious. You can bet your boots that like Brother she spends big chunks of her life wishing she could go back in time and do things differently. Like Brother, she probably remembers that Doodle didn't look right and kicks herself for not addressing the problem.
In terms of character, Mama is rather sparsely developed. We see her cry a few times. We know she makes a mean peach cobbler, and that she makes her kids ask to be excused before they leave the table. Part of this is because the story is told through Brother's eyes, and he and Doodle are the big focus. So, it's up to us, or, rather, you, to imagine her story. If she were the narrator, how might the story be different? How does she feel about Brother? How do you know? How does Brother feel about her? How might Doodle feel about her? What is her relationship like with her husband? Are there answers to these questions in the text, or not?