If Sam's mom is one thing, it's proud to be a witch. That's one reason why being married to Kevin was a lose-lose situation—as Nick warned her when she went to visit him about her family tree project, "If you stay with him, you're going to have to hide who you are" (12.35). We've got to give her credit for trying, but in the witch department, Tia can't fake it 'til she makes it.
After all, Sam does say that she's "not a big fan of the straight path" (11.1). All you have to do is look at her house to see that there's something a little off with this lady. Her front yard is a garden, but even that doesn't do it justice. "You can't really fully grasp it until you open the gate and walk on the cobblestone path through what one mailman described as the 'forests of LaCroix" (11.2). We can't be too surprised by this; after all, true to the witch stereotype, she's a big herbalist and a general fan of using naturally occurring elements for her spells.
As if being a witch doesn't have its own set of complications, the really complex thing about Tia is her relationship with Sam. The protections she takes at his birth to keep Douglas from discovering his powers and to keep Sam from evil in general demonstrate that she loves her son deeply, but it's clear to Sam that she, at least on some level, fears his power. It's a fear he senses after she tells him the story of Nick and his biding, and it hurts him. Bad. Sam reflects:
The idea that my mother, the one person who was supposed to love me without reservation, could fear me made my stomach clench up.
Based on events later in the book, we're pretty sure that Tia recognizes and regrets this flaw in her character. When Mrs. W. and Ramon arrive at her house with instructions from June Walker to remove Sam's binding, she's initially reluctant. "I don't know if I can," she tells them. "He's so far away" (22.126). We're pretty sure that Tia isn't just talking distance here—she seems to sense how her reaction to Sam's gift has hurt him, and these emotions may even be clouding the way to removing the binding and saving Sam.
By the way, if you still think Tia's an ice queen (or witch) for distancing herself from Sam, check out their reunion later in the book after Douglas's demise. Tia apologizes to Sam multiple times through choked sobs, and the two reconcile. "We're going to start over," Sam agrees. "What's done is done […] And we're both going to have to live with that" (30.123). Onward and upward, you two.