I think that it is very possible to be a bad parent but a good person. Parenting is a skill. And some people find it easier than others. Just like some people are good at certain things only or some people are smarter than other people.
Also, parenting doesn't happen in a vacuum. in the case of my PIL, there were a LOT of other things going on - MIL has, I believe, almost crippling ADHD. She also clearly suffered from significant mental health issues that were, of course, unacknowledged or dealt with. FIL had had a very difficult childhood of the sort that today, any child in that situation, would be given a lot of extra support and care (lots of close family dying, often in very difficult ways, with him getting a front row seat to all of it as a resutl of culture, poverty and the time...). It meant that he had almost zero ability to express or manage emotions and that earning money to ensure him and his family would be "safe" was almost the most important thing.
they were both immigrants in a country where they didn't (at first) speak the language. They had vey little money and had to support, practically and financially, elderly relatives.
Movies and books show us families like this and how the parents pull together and still find amazing ways to be present for their children, and do amazing things to bring them up.... but I don't think in the real world, it's always so easy. And one of the reasons' DH is the favourite is because it was easier ofr him to fit into this life they were trying to build - he is naturally outgoing and smart and good at languages. So he started school speaking pretty good English already, whereas his siblings didn't, and he quickly made friends in a way his siblings found harder. He made them "look good" because he was good at sport and at music and got praise at school for it. His friends were from "good" families.... etc etc etc.
PIL did the best that they could. But it wasn't really enough.