Since David's dad left them a number of years ago, David's mom is responsible for bringing up David on her own. She works as a pharmacist, and according to David, "She thinks there's a pill for everything" (3.30). We also learn that she's diabetic (10.71), which probably contributes to her faith in modern medicine.
She's not what Laurel expects from a mother, either: "She was wearing a cute red tank top and jeans and her long, almost-black hair was loose and tumbling down her back in waves. She was the most unmotherly mom Laurel had ever met. In a good way" (6.10). Whenever Laurel comes over to study, David's mom offers them snacks, but also stays out of their hair.
According to David, this is because: "I've earned a lot of privacy by not asking why a lot of my mom's dates don't leave until morning. I stay out of Mom's personal business; she stays out of mine" (6.19). Doesn't sound like a bad deal to us, and hey, just because David's mom is a mom doesn't mean she can't have a social life.