Is mythology considered on-topic for this site? I'd like to how the History.SE community feels about this (personally, I think it's off-topic).
Since Mythology is more of a set of stories without a hard basis that can be answered factually, unless about content but origination would be hard on many, I'd say it doesn't belong in History SE. If the Mythology can be put in a historical context (maybe something like - who were the story tellers of the Gauls and how were they known to impart their knowledge?) then you might be able to get an answer on some sources. As to the myths themselves they might have a basis but that might be beyond the reach of the site and its purpose.
Mythology is, obviously, not on topic here, but it is now on topic on the recently public Mythology Stack Exchange.
Feel free to refer people to the new stack!
The sort of mythology involving gods, monsters and superhuman feats should be clearly off-topic.
But there's also the mythology concerning more worldly affairs such as the establishment of cities and the rule of various heroes and sage kings, and as they are related in proper historical works by the likes of Plutarch and Sima Qian, it is not always easy to see where myth ends and history starts. I think that the historicity of such events is best left to answerers and questions on these subjects should be treated as on topic.