Aside from the social biases she faces as a black woman, I don't see that Calpurnia has any major limitations as a mother figure. After all, Cal is a mother and a grandmother and may even be approaching the status of a great-grandmother--her son, Zeebo, "had half-grown children." Cal has received some education, and she has seen that Zeebo has learned to read and write; they are two of the only members of their church congregation who can do so. Cal has a prominent position in the eyes of her friends, housekeeping for Atticus--the black man's best white friend in Maycomb. Zeebo, too, has a full-time job as a garbage collector. So Calpurnia has done a fine job for herself while successfully bringing up her son. Having grown up with Atticus at Finch's Landing, Cal's character is unquestioned, and after Atticus marries, he takes Cal with him to Maycomb to help his wife with their children. After his wife's death, Atticus realizes that he already has the right person to help raise his kids. Although Cal is nearsighted and "her tyrannical presence" often angers Scout, Atticus considers Cal
"... a faithful member of this family.
"I don't think the children have suffered one bit from her having brought them up. If anything, she's been harder on them in some ways than a mother would have been... she's never let them get away with anything, she's never indulged them the way most colored nurses do... and another thing, the children love her." (Chapter 14)