The misconception may stem from the fact that they have thick fur covering their entire bodies, including their rear end. This fur can make them appear to have a padded bottom, especially when they are sitting down.
Their thick fur serves as insulation, keeping them warm in the cold Australian nights. It also helps them camouflage in the trees, where they spend most of their time.
Instead of padded butts, koalas have strong, muscular hind legs that help them climb and balance in trees. They also have sharp claws for gripping branches.