Let's begin by reviewing the difference between a dynamic character and a static character. Dynamic characters change significantly throughout the story. While they might experience external changes, their true dynamism is found inside them. They learn important lessons through their experiences and become significantly different people by the end of the story. They might change for better or for worse, but by the concluding lines, they are not the people they were at the opening.
Static characters, on the other hand, do not experience such significant internal changes. They stay pretty much the same; they are set in their personalities and actions, for good or for bad. This does not mean that static characters are boring characters. In fact, many static characters are also what we call round characters—characters that are interesting and complex and have many layers to their personalities and complicated motives in their actions.
With that distinction in place, let's think about Lena Younger, also known as Mama, in A Raisin in the Sun. We can observe that Mama does not really change all that much throughout the play. Her values and her personality are settled. She is a strong, faith-filled woman who loves her family (even in its messiness) and wants a better life for the people she loves. She has a dream of owning a home in which her family can have a better life. These characteristics and this dream do not change throughout the play.
Mama might surprise us sometimes, like when she slaps Beneatha for her ranting about not believing in God or when she gives Walter the remainder of the insurance money, but these incidents do not really reveal a change in Mama's character. Rather, they show the depths of her personality, the strength of her faith, and her love for her children. By the end of the play, the Mama who was set on buying a new home at the beginning is the same Mama who is ready to move in at the end. Mama is, therefore, a static character.