Slackware was themed after the Church of the Subgenius, which is a satirical religion and online movement about "slack" as a cosmic force. It came from a time when software projects could be more whimsical and show some character. It's almost impossible to imagine a software project today expressing any kind of off-topic flair, or really be eccentric in that kind of way. There's hardly anything like it today, but the appeal of something so quirky I think is still easily understood.

Edit: I think at one point there was a Linux distro based on My Little Pony, is that right? It was a different time, you had a new space that had just opened up for creative exploration, and it was accessible mostly to nerds and people on the fringe. There were a lot of weird things coming out back then.