In addition to describing the narrative point of view as first person, the narrator -- Mama -- is also a subjective narrator: the point of view is first person subjective. What this means is that, in addition to being a participant in the story and using first person pronouns (like I or we), Mama is also narrating the story in the present: as it happens, and not after the fact. You can tell because, when she describes the action, she uses verbs that are in the present tense. For example, she says, "'Come out into the yard,' I say," when she describes talking to Maggie before Dee/Wangero arrives.
First person subjective narrators tend to be somewhat less reliable than first person objective narrators (who narrate in the past tense, after all the action has taken place), because they've had no time to reflect on events. We get their immediate, sometimes emotional, often unfiltered, responses. In a sense, we see these narrators' immediate and truthful reactions, but we don't benefit from their lack of time time to synthesize everything that's happened. With Mama, such a point of view seems to make sense, though. She doesn't try to arrive at any conclusions about the interactions she's had: she takes them at face value, reports them, reports her responses to them, and that's the end of it. The audience is left to draw their own conclusions.