The sloth is well-named. It is lazy and slow. There are flowers that bloom faster than a sloth moves. No kidding.
Let me say some nice things about being slow. Slow can work. Tortoises are slow. Snails are very slow. Tortoises and snails are extremely successful creatures.
What make the sloth an example of Dumbass Design™ is that sloths are warm blooded. Being warm blooded takes energy, and the advantage is that a warm blooded creature can stay active in a greater range of temperatures. Most cold blooded creatures become nearly motionless when the weather gets cold, some reverting to a hibernation that borders on suspended animation.
Tree sloths live in tropical rain forests, where the temperature stays hot enough for even cold blooded creatures to stay active year long. So we have a warm blooded creature moving as slow as a cold blooded creature in an environment where even cold blooded creatures don't have to be slow moving. Energy expended for no advantage whatsoever.
The Dumbass Designer™ moves in mysterious ways, his blunders to perform.