In their own way, each member of the Younger family is a protagonist in that each of their prime motivations is a better life for themselves and their family. The real antagonist of the play is the force of institutional racism and the specific practice of housing discrimination represented in the play by Karl Lindner and the Clybourne Park Improvement Association. However, it is possible to consider particular members of the Younger family as antagonists in that they have relatively more selfish incentives and more flawed means of pursuing them.
By this logic, Mama is undoubtedly a protagonist. Above all, she wants a better environment to nurture the future generations of her family. In the play, Mama's hope for the future is symbolized by her attachment to her plant. Her desire for a garden of her own symbolizes a space where the Younger family can thrive and grow in proper soil and with proper care. Out of love for her son Walter, though, she is willing to compromise her notion of the family's best interest in order to grant him an opportunity to be the "man of the house."
Conversely, Walter is arguably an antagonist because—while the particular shape of his aspirations are conditioned by the limitations of a racist society—he places his individual aspirations above the best interests of the family. Walter's decision to invest the family's legacy money in a liquor store signals misplaced values, especially given the fact that he has chosen partners in the business venture that prove less than trustworthy.