Ma is the book's first casualty, but we'd argue that she is gone long before she actually dies. She's a victim of the 1950s society that puts women in their place (i.e. thanks, woman, now get us a sandwich) and it's a place she doesn't really want to be.
Things really start sliding for Ma when she has a miscarriage. That would be tough for any woman, but Ma doesn't have the resources to cope with it. She doesn't seem to have friends (note the similarity with her daughter) and she has a tendency to try to replace things—in this case, Ma replaces her dead son with a parrot named Petey. She even kisses the bird on the beak. Her attachment to Petey makes Dolores jealous (then again, though, what doesn't?) and she has a "nervous breakdown" (3.1) after the parrot dies.
When Ma gets out of the hospital, she gets a job and starts dating. Dolores isn't happy about the latter, mainly because Ma seems to be getting attention from men, something Dolores doesn't get. After Dolores is raped, Ma buys her tons of food and a TV, which isn't the healthiest way of dealing with things. The underlying message is tune out and shut-up.
She basically enables her daughter's inertia, and makes it that much harder for Dolores to recover. By the time Ma takes a stand against Dolores (the monster that, frankly, she created) by cutting the TV power cord with a steak knife, it's too little, too late. She says to Dolores, "I will get this fixed… when and if you have that physical and get that form signed. I happen to believe in your future" (8.30). Hmm… she didn't care about her daughter's future at all when she was helping Dolores balloon up to 257 pounds.
Dolores fiercely resists the whole college thing—at least partially because she's been sent a message by her mother that she's not particularly valuable for years—which prompts Ma to say "You've made me so goddamned tired" (9.18). This is the last thing she says to her daughter. She goes to work at her tollbooth, where a trucker falls asleep at the wheel, crashes into her, and kills her.
Death sometimes turns people into saints, and Dolores's attitude toward her mother is no different. Dolores feels responsible for her mother's death, as if not being a horrible teenager would have kept the truck driver awake, and in feeling so, changes her tune about Ma. Dolores even tells Dr. Shaw that her mother was "a saint" (17.134), which he rightly disagrees with, working with Dolores to help her develop a more realistic picture of her mother: flawed, human, a questionable artist, but someone who still loved her.
In life, Ma isn't a saint but instead "a fragile woman, a victim in many ways" (19.1). Before she dies, she does her fair share of damage to Dolores, but considering how damaged she is herself, it's hard to blame her too much.