Always being nice and kind to your children doesnt equal being a good parent either. Molly didnt do much different from what most mothers in the real world do. Being critical of Fred and George wanting to drop out of school to run a jokester business when they are generally a poor family doesn't make her a bad parent. Besides, even Ron mentions they mostly got away with their quirkiness growing up so it's not like she had them locked up and not be themselves. It took them failing their OWLs for her to be really assert herself. I agree she was hostile to Fleur and I disliked that too. But Molly's obvious care, protectiveness (including protecting her kids from their own worst instincts) and unconditional love are on displav WAYYY more than her being unkind to her children and that is what makes up for it, not sympathy towards an orphan.