I have this theory that sloths are slow and sleepy because they're actually an alien species that were left behind and have lost contact with the mothership. Once the mothership returns, the sloth uprising will come.

Your theory is almost right. More specifically, the Sloth Overlords (more commonly known as space sloths) are transcended being rather than from a different planet, although being transcended may seem “alien”. With this in mind, the sloths merely need to “wake up” their minds to be a space sloth. Also, we are too lazy to create an uprising. We’d rather be napping or eating. 

Thank you for your thoughts human. We hope this clarifies things. May the Sloth Overlords watch over you.