Clearly questions about Zeus or Thor are mythology and appear relevant to the mythology stackexchange. Stories about Paul Bunyan or the Yeti probably aren't relevant. What is the dividing line between Mythology and just stories?
There may not be any need to define the line. Would the site really be hurt or diluted if folklore was within its scope?
If you want to look at definitions, here are some Wikipedia definitions of mythology and folklore:
According to Alan Dundes, a myth is a sacred narrative explaining how the world and humankind assumed their present form, although, in a very broad sense, the word can refer to any traditional story. Bruce Lincoln defines myth as "ideology in narrative form". Myths may arise as either truthful depictions or overelaborated accounts of historical events, as allegory for or personification of natural phenomena, or as an explanation of ritual. They are transmitted to convey religious or idealized experience, to establish behavioral models, and to teach.
Folklore (or lore) consists of legends, music, oral history, proverbs, jokes, popular beliefs, fairy tales, stories, tall tales, and customs that are the traditions of a culture, subculture, or group. It is also the set of practices through which those expressive genres are shared
Folklore can be divided into four areas of study: artifact (such as voodoo dolls), describable and transmissible entity (oral tradition), culture, and behavior (rituals). These areas do not stand alone, however, as often a particular item or element may fit into more than one of these areas.
I think some of the lines will have to be drawn on a case-by-case basis, especially because, as the Folklore Wikipedia page says,
In usage, there is a continuum between folklore and mythology. Stith Thompson made a major attempt to index the motifs of both folklore and mythology, providing an outline into which new motifs can be placed, and scholars can keep track of all older motifs.
At least in my opinion, one of the main lines should be drawn is that a creation story is considered a myth, and as such, any story or set of stories spanning from that creation story is considered part of that mythology. For instance, anything spanning from the Greek pantheon (including human or half-human participants in stories that include the members of that pantheon) would be considered in that mythology. Stories that are not at all related in any manner to a creation story should generally not be considered mythology.
Also, see: What should define Mythology?
When I saw the proposal for a mythology Q & A, I presumed it would be a place to ask mostly about tales rooted in religious and origins or those that were at some time considered sacred by a culture: Dionysus, Jesus of Nazareth, or Valhalla.
But then, how could we rule out that this site will invite questions regarding mythological creatures, like vampires, dragons, and fairies. Many of which are introduced by folklore.
A hard line may be difficult, but it seems that a strong guiding point would be to consider the presence of a strong historical and cultural influence as opposed to something like modern pop culture.
This line might be approached, for example, on the topic of vampires. According to wikipedia:
Tales of supernatural beings consuming the blood or flesh of the living have been found in nearly every culture around the world for many centuries.
That seems pretty straightforward, but what about the character Dracula? This might seem a bit fuzzy as it is of more recent historical and cultural influence. But nevertheless, the character has had such a huge cultural influence that it survives to this day as an archetype well beyond that of its original conception.
The connection of the name "Dracula" with vampirism was made by Bram Stoker around the 1890s Since then, "Count Dracula" has been a recurring character in vampire mythology and media.
The same can be said for some of J. R. R. Tolkien's work, itself influenced greatly by European mythology and Catholicism1, whose influence permeates much of the fantasy genre today with elves, dwarves, goblins, and other creatures being common elements that have each taken on a sort live of their own. Although there is already the more specific Science Fiction & Fantasy network, there certainly seems to be some room for exception when you consider that people may have questions surrounding the mythological origins of dragons or goblins. Specific questions pertaining to the context of Lord of the Rings would not fit in well here at all.
These last two examples probably most closely approach the limits of what constitutes as mythology. Going back to vampires, a more clear example of what, to me, doesn't seem appropriate for this network would be a question regarding Edward from Twilight or the comic book character, Blade. While these are extensions of mythological roots regarding vampires, they do not themselves extend far enough beyond their original context so as to have become strong cultural archetypes. They would really need to tie the question to mythology somehow, such has "How does Edward compare to the typical mythology of the vampire?" or something that requires more knowledge of the mythology and lore of vampires than it does the ability to quote the Twilight movies line for line.
Dracula, as he exists in the broader context in culture, may warrant interesting questions, so long as they pertain to the common myths surrounding the character. Blade and Edward on the other hand are not so interesting in this sense as both are have quite a narrow, centralized source of origin, from the minds of one or very few individuals. Nor has either of these grown to become significantly reoccurring representations of an archetype. I would downvote anyone trying to pass them off as "mythological."
On the other hand, a question about Dracula would require more of per question judgement. If someone wants to know about a particular work of fiction, there are more appropriate places for that than a mythology Q & A. If someone asks "What supernatural powers are commonly attributed to Dracula?" this seems more appropriate as it asks about the commonly held lore of the character. Right away you're probably thinking that he is said to be immortal, can shapeshift into animals, and has a supernatural ability to exert his will over others. While these attributes may be supported by specific works of fiction, they are all things that people "just know" without ever really having to reference them.
One must consider that all of these tales were at one time, just that. Stories. To me, the presence of broad social and cultural influence, especially in a religious context, is the strongest indication of relevance to this Q & A.