Who is Maureen Peal? What does she represent in the novel?

Maureen Peal represents the problem of colorism, which exists in the black community but is not limited to it. Colorism is the discrimination that can occur among a race or ethnic group due to the tendency to prescribe value, usually, to those who have lighter skin.
Maureen is a little girl who knows that she has social value and she knows why: black people who have proximity to whiteness are deemed more beautiful and respectable than those who don't.
Maureen exists in the novel as a foil to Pecola Breedlove. This isn't merely because Pecola is very dark-skinned and Maureen is very light-skinned but also because Maureen is, as the previous educator noted, popular and charismatic, as well as intimidating (the narrator, Claudia, notes how the boys' eyes "genuflect" when they are around her), and vain. Everyone wants to be around her and Morrison reflects this in her name, Peal, which is the clear beckoning of a bell. Pecola is much nicer than Maureen, but no amount of sweetness or desire to be loved draws anyone toward her. Maureen's clothing is described as fashionable and neat, while Pecola's aren't. Maureen's hair curls in ringlets; Pecola's tightly-curled hair is in plaits that would be described as "nappy."
Claudia dislikes Maureen but also wants her approval. During a conversation about which Hollywood stars they love the most. Claudia expresses her distaste for Shirley Temple, whose saccharine-sweet image and ringlets remind her of Maureen. She expresses a preference instead, for the exotic beauty of Hedy Lamarr. The girls' friendship with Maureen ends with Maureen mocking them for being darker-skinned and acting as though she was doing them a favor by associating with them.
Maureen is one of the most distasteful characters in the novel, but it's easier to forgive her than, say, Geraldine, because she's a child. However, the fact that she's already internalized the tenets of racism, which teach her to believe that she is prettier, more lovable, and better than Claudia, Frieda, and Pecola because she's lighter-skinned reveals how pervasive and corrosive white supremacy is.

Maureen Peal is the new girl in school. She's attractive, charismatic, popular, and comes from a good family. Most significantly of all, however, she also happens to be light-skinned. This means that she approaches more closely to white society's notion of what is beautiful.
Maureen's introduction in the novel reinforces a number of important themes. For one thing, Maureen instills feelings of self-loathing in Claudia, who comes to hate herself on account of her dark skin. Maureen's so much better, she thinks, and all because of light skin.
Morrison's making a general point here. The prevailing standards of beauty in white society lead to African Americans not accepting themselves and their skin color. In this society, white is considered good and black bad. The only way that African Americans can become more accepted in this society is if they display the same kind of characteristics associated with white people, whether it's light skin, wealth, or high social status, all of which are possessed by Maureen Peal.