How do we know they aren't concentrating on filling themselves with light and pondering the meaning of life as they lie there? We don't. They may know the meaning of life, which is why they look so relaxed. We don't speak Sloth so who knows?
Sloths do enough to feed themselves when they need to eat; reproduce to maintain the species and care for their young.
My feeling is they are scapegoats. There's nothing wrong with being chilled. As long as we do all we can to look after ourselves, and any dependents, then that's a good start.
If we are able to do that without getting stressed, running round in circles until we are worn out and "fit for nothing else", all the better.
Relax when you can ... be MORE like a sloth.