We never meet Mattie's mother, but we get some hints about her. Compared to Mattie, she may seem weak—but Mattie doesn't see it that way. Mattie's mother is simply a different kind of woman, the way Frank Ross is a different kind of man from Rooster and LaBoeuf.
Compared to Mattie, Mrs. Ross chose the traditional path of marriage and family; Mattie describes her as a "Mary," who had "chosen the 'good' part" (1.11). (See our "Character Analysis" for Mattie for more on this Biblical reference to Mary and Martha.)
Mrs. Ross may be a demure, gentle person who probably never recovered from the death of her beloved husband. But weak? Not so much. She watches her daughter's arm being amputated without breaking down—which sounds pretty strong to us.