I don’t understand your question. If your novel uses existing mythology, it is perfectly fine. If you invent your own, it is fine too. In terms of conveying the myths in the story, the main concern would be to avoid the obvious and boring infodumps, such as “as you know, Bob” or a long lecture by a character. Usually, the best way is to have it in your notes, and let it drive or motivate your characters, without copy-pasting all of your setting notes into the novel.
