When Katniss lost her father, she dug in tighter. Her mother, on the other hand, just kind of gave up on life. She stopped doing those little mom things that make such a difference, like feeding her children on a regular basis. That forced Katniss to grow up in a hurry, which might have given her the toughness she needed to survive in the Games.
Like Primrose, Katniss's mom works better as a way of explaining who Katniss is rather than being a stand-alone character herself. But she's worth noting because of who she helps Katniss become (however accidentally) and where that causes the story to go.