Ma is dead by the beginning of the novel, having succumbed to breast cancer, but we do learn a bit about her over the course of the book. She was raised by a single father, and ended up taking care of him as he aged. She also had a harelip she was intensely self-conscious of, often covering her face on the rare occasion she was photographed. Her insecurity was at least partially thanks to her abusive father's fondness for talking about how homely she was.
Even though her father made her eat cigarettes and regularly hit him, she worshipped the ground he walked on, to the point where Dominick wonders if his grandfather is also his father. See, despite birthing and raising twin sons, she won't tell them who their dad is. Or not Dominick, anyway—we eventually find out Thomas is in the loop.
Ma married Ray, who supported the family in the way many 1950s dads did: by working and paying the bills. He was also abusive and cruel to his wife and her kids, and felt he was entitled to be, because it's like they were even his kids. For her part, Ma was submissive, letting Ray do whatever he wanted to the kids and not interfering.
Dominick forgives her for this, but he is angry with her for favoring Thomas over him. He's also angry because he worries her tendency to shelter Thomas, or to dress him up for tea parties, played a large part in his mental illness. But at the same time, he's also angry because he wanted her attention, too.
Unfortunately, we never get Ma's side of the story from her own mouth. She rarely talks about her past when she's alive, and once she's dead, all Dominick can do is speculate.