Those mythologies (just like the Middle Earth mythology based on Tolkien's writing, or the Discworld mythology in Terry Pratchett's writings) are generally on topic in the Science Fiction and Fantasy Stack Exchange site, so we wouldn't lose much if we don't allow them here, as long as we clearly direct people to that site in the help text.
Just wanted to make a couple of additional points:
Real mythologies and fictional mythologies (by the way the word is "mythopoeia") though having clearly distinct origins share some of their purposes: conveying a vision of what the world is and why it is like it is, and building a moral or philosophical framework. So in this sense it would actually make sense to discuss them here as well.
These mythologies, though fictional, borrow heavily from traditional mythologies and someone wanting to understand what part comes from the author and what part comes from actual mythologies will be better served asking the question here than on SciFi.
Mythology is mythology whether it exists solely in a fictional world or existed as a form of worship for actual people. So long as the question is about the mythology and not about the fictional work it should be on topic here.