You can't just ask that question, it's pointless. The "cause" of every bully is different. Without context no-one can say whether YABU or not. It could be rubbish parenting, but there could be other, underlying reasons. For example:
Despite parents trying to access help, the child has undiagnosed issues such as autism or aspergers that means they are not coping well in the school environment and are lashing out = NOT shit parenting.
Little Jimmy or Susan is a spoilt child who is given everything they want at home and is allowed to do what they want so think they are entitled to push another child over and snatch their stuff = shit parenting.
A bit like Florieboo, but not mine, I know a lovely family of three kids, two are kind and sweet, the other could be a hideous, mean bully when she was younger. She is growing out of it now thank goodness but she was struggling with all sorts of things, including an in-built, terrible temper which until she began to learn to control it did make her quite awful!