As the owner and principal employee of Jesus Is Lord Used Tires, Mattie is unlike any woman that Taylor has ever known. When Taylor first meets her, Mattie is wearing "blue jeans and cowboy boots and a red bandana on her head," and she has "a long gypsy braid" that hangs "down the middle of her back" (3.33). Imagine what Katniss Everdeen might look like at fifty years old—that is, if she ran a used tire lot in Tucson.
Mattie didn't name Jesus Is Lord Used Tires, but it's clear that she's a Christian woman. For one thing, she's fond of attributing weather and blessings to God. As Taylor tells us, Mattie insists that:
"The old-timers, somewhere down the line, would look back on this as the year we didn't have a winter, except for that freeze God sent on Valentine's Day so we'd have green-tomato pie. When the summer wildflowers started blooming before Easter, Mattie said the Lord was clearly telling us to head for the hills and have us a picnic. You never could tell about Mattie's version of the Lord. Mainly, He was just one damn thing after another." (7.9)
Although Taylor never says it explicitly, the likelihood is that it's through Mattie's connections to local Christian churches—like Father Williams'—that she gets involved with the "underground railroad" that hides Central American refugees in safe houses throughout the country. Whatever the specifics of her faith, she's committed to helping the people she hosts in her "sanctuary" above the garage, and also to raising awareness by speaking out about America's responsibility to shelter refugees.
More than any other person in Tucson, Mattie becomes Taylor's mentor and role model. As such, she stands in as a surrogate Mama to Taylor when the younger woman goes through her roughest and darkest times.