Sarah Lynn actually did lack agency being a child actor and facing constant abuse from family and outsiders (see Britney spears). Also, Diane or Sarah Lynn aren’t considered that bad because their actions mostly don’t affect others as much as Bojack’s do. Bojack’s also has a lot of trauma that is explored in the show as well, helping people empathise with him but his self destructive streak actively destroys the lives of others without much consequences for him