Why is Kaeloo so upset about Stumpy and Quack Quack being annoyed with their parents? She's Quack Quack's Parental Substitute.
Given Stumpy's tendency to exaggerate things, he must have told Quack Quack that parents are horrible and evil, and being an orphan, Quack Quack would have no way of knowing if that wasn't true. His empathetic nature would make him likely to want to support a cause which doesn't directly affect him.
Fridge Horror
Quack Quack's presence at an anti-parent protest seems awkward until you consider the possibility that he's trying to convince himself that all parents are horrible so he doesn't feel bad about being an orphan.
When Mr. Cat sees Kaeloo yelling at Stumpy, he informs her that yelling at a kid can traumatize them for life. Mr. Cat has mentioned before that his own mother used to yell at him a lot when he was a kid.
An exhausted Stumpy falls asleep at the breakfast table before going to school, and his face lands in his cereal bowl. If Kaeloo hadn't woken him up, he could have drowned in the milk.
