This is an extremely late reply ^^; but sure here’s why koalas are the absolute worst animal ever :
Firstly most koalas have an STI called chlamydia, which the koala can give to up which is pretty bad considering you can never fully get rid of an STI.
Then there’s it’s eating habits, it only ever eats eucalyptus which is acidic and erodes their teeth so they can eat anymore, eucalyptus is also not very nutritional and domes give the koala enough energy so if you ever see a koala fall out of a tree it’s probably because it ate it’s fill of eucalyptus and didn’t get enough energy from it and fell unconscious. Obviously animal preservers tried to help the koala by trying to get to eat eucalyptus in different forms like a paste or a liquid so they could continue eating even after their teeth eroded away, but koalas being the dumbest creature alive refused to eat it in any other form. The animal preservers then breaded koalas that ate other food this was great for the koalas but they desecrated habitats for other creatures. Thankfully all of these koalas that ate other types of food than eucalyptus died in bushfires. And as you said koalas feed their baby’s with their own poop or feces this contains even less nutrients because it was absorbed by the parents body, they do this because like birds the baby koalas can’t chew the food though the koalas way of doing it is hella lot less productive.