I think mythology works better on the open mic or in a feature than in a slam. I lay the blame squarely at the feet of the three-minute rule.
If I'm working within established mythology (Zeus & Hera, Jesus & Peter, or even Huckleberry & Jim) then my poem is going to live or die on whether those 5 judges have read ancient Greek, the Bible, or Mark Twain. Some people are going to think I'm elitist and snobby: buddy, are you telling us a story or just showing off your collection of leather-bound books?
If I'm making up my own mythology, then I have to explain to you what a Freezleump is, why the city of Zionimoo is warring with the Kingdom of Yat, and why the Doctrine of Arcturius Kye is going to pressure my protagonist into self-defeating behavior. Okay, I'm a writer, I can do that, but laying down that kind of groundwork isn't going to leave me much time to tell my story.
I think that's why we see so little sci-fi, mythology, historical fiction, etc in slam. It's way easier to say "I'm me and this week I met a girl with freckles and this is my poem about that." Took me less than five seconds to get all the judges on the same page. And that's the easy way out and it's such a tempting rut to get stuck in but that's why so many of us get stuck in it. Open mic is a different story. Some people on the open mic come back week after week to tell an extended story. First-timers might be a little lost, but the regulars are on board and that's a choice those writers have made.
(For what it's worth, I knew the Yat King was bad news the minute I laid eyes on him.)