Over the course of the book, as we learn more about what's really going on, our view of Mum changes dramatically. At first she seems a lot like the crazy-woman-in-the-attic type so typical of Gothic novels, and her physical description doesn't help dispel that image:
Her broad slanted cheekbones made her look catlike, and her wide mouth was constantly moving and always formed a shape at odds with her expression. She was tall, and her black hair made her look even paler than she was. […] Past the protection of hooded eyelids that seemed pulled down by heavy lashes, her eyes were icy blue and seemed always fixed on someone who was not there. (3.101)
Um, yeah—that's crazy to a T. But the more we read, the more we know, and the more we realize Mum is just one of the people/events we see only through Nick's eyes, which also makes Mum one more of the people/events that Nick misunderstands.
Don't forget: other people (Alan and Mae) seem to get along with her just fine, and as Nick finally realizes when he goes to her room to speak with her, "he always thought of her at her worst, during the screaming fits or the times she had to be medicated. She was always at her worst when Nick was there" (12.27).
This makes sense, of course, because presumably every time she sees Nick she relives a major trauma. But we don't know this until late in the book, and when we figure it out, we begin to see Mum's character as just one more example of how Sarah Rees Brennan's expertly plotted tale has managed to turn us completely upside down.