I truly think she was a good mom. She was always there for her daughter. In the early years, she was actively learning about how to be a good parent, reading articles and books. Trying to practice good parenting. It's not her fault that Buffy had the tendency to jump out her bedroom window. And, arguably, she became a surrogate mother to each of the Scoobies, which is why her death hit so hard in season 5. It wasn't just Buffy who was grieving, but the whole gang. While we didn't always see it on screen, I imagine Joyce had built positive maternal relationships with each of the Scoobies by season 5. Willow and Xander had crappy parents, who were constantly absent or abusive. Joyce filled that role for them, while she was alive.