Mclean's mom rarely appears in person in the book because Mclean really doesn't want to see her—in fact, she tries to be in the same physical location as her mom as little as possible—but that doesn't mean that she's not a huge driving force in the story. After all, most of Mclean's hurt and pain comes from what her mom did in tearing their family apart:
Ever since the divorce, and my ensuing that I did in fact have a choice and an opinion concerning it, I'd justified every bit of my anger toward my mom simply because of how she'd wrecked my dad. (6.197)
And although her mom won't acknowledge what she did in the way that Mclean needs her to, she still tries to reach out to her daughter constantly. Even though Mclean is as bristly as a porcupine with her, she still calls all the time and tries to get Mclean to visit so that they can have some of their old closeness.
Mclean's mom may have a brand new life, and it may be shinier on the outside than her old life, but that doesn't mean that she wants to leave her daughter behind forever—she still wants to be a part of her life, and she's not going to stop calling her until she lets her back in.