I think the reason for the Owl-hate is similar to the reason some people really hated Diane in BoJack Horseman. People feel like Owl "should" be better than he is. He's the most functional of the trio - he holds down a job, cares whether the house is clean, he does far fewer drugs than Megg and Mogg. He should've left that pit of horrible a long time ago. Instead he hung around for almost a decade after high school, perhaps because he has a crush on Megg still, but in my opinion because living with two huge drug addicts allows him to indulge his own cravings while handily blaming them when the shit hits the fan. Owl, you know you have to be up early for your new job working for your dad's golfing buddy, WHY are you doing ketamine with the two stoners?! You're the only one who cares about bettering himself so what the hell?
Megg didn't have much of a chance in life, her mother dealt drugs out of the house while she was a kid (something nobody would do if they had a decent income), and Mogg is quite content to leech off his parents. The bar is much lower for them because realistically, what can you expect from them? Owl is capable of better behaviour and of escaping Drug World, and so people shit on him harder.
