Esperanza narrates the House on Mango Street completely from her own point of view. She's very honest about her emotions and doesn't free us from her feelings of guilt and shame, but much of the novel is bred in her imagination........ let's face it, she details things throughout that she has never seen, and that she couldn't have seen because she wasn't there.
Ultimately, Esperanza is a writer, and she is endowed with a very healthy imagination. Her description of Marin under the streetlight (Chapter 11) is fabricated, as is the domesticated scene she supplies us with in "The Family of Little Feet." Even when Rafaela is trapped in her house alone at night, Esperanza doesn't know what the girl is thinking, but she can imagine it, and she does.
In the beginning we have this same type of narration in regards to the house she dreams of, the one she'll own someday. I don't think it's her intention to distort, only her desire. But is she unreliable? Yes, never believe everything you read.