Nana has had a hard life, and it has made her into a hard woman. It's hard to look at her story and not feel compassion for her struggles, though; after all, she raised Mariam all by herself—most wouldn't even have done that.
The big elephant in the room when it comes to Nana is the jinn that "entered Nana's body," which can be more accurately defined as an epileptic condition (1.2.15). It's notable that Nana refers to her medical condition as a jinn (which can be roughly translated as "genie" or "spirit") because it shows how limited her own education has been. She's obviously a perceptive woman, but she lacks the education to truly make sense of the things that happen to her.
Nana comes from a poor family, which further complicates things. It's doubtful that her family could have afforded to get Nana the care she needed, which could have contributed to her medical condition. Likewise, Jalil comes from a higher social class than she does, which pretty much gives him the power to stomp all over her.
Nana has issues: you can't argue with that. But she did her best, and she did it honestly. That's more than could be said about many of the fathers of the novel, like Jalil or Rasheed.