Thinking about the aspects that have some interplay with folk horror.
The big one, obviously, is with urban legend as modern folklore thing, which I tend to side with, though I suspect an internally consistent argument for the opposite assertion could be made. (A big part of the question in my mind would be "what is folk and how much role can the mass media play in the propagation of folk culture before it is no longer "folk"")
But to screw down to questions relating to this specific phenomenon (though ones I feel will be less capable of providing answers to the initial question).
What is the importance of the mask in folk horror? There's something there from the ritual aspect, the taking on of roles, the masking/face blackening of mummers/trick or treaters/ performative travelling (door to door) entertainers/beggars of all sorts. How the mask allows people you know to act in ways normally considered anti-social, or take on a status outside of the one they occupy in the community.
The question I guess is how specific is the horror of the mask to folk horror as opposed to all horror, and is its meaning in this case in line with its meanings within the genre?
The clown as a folk horror figure is tied to the carnival or travelling circus as setting/catalyst for horror (which obviously has nothing to do with the current panic, just establishing that the clown as horror figure does have a place in the genre). When a clown appears as a lone figure (or a figure divorced from the confusion of the circus/carnival) in horror it's usually more as a agent of cosmic horror, fundamentally unseating previously solid assumptions about how the universe functions held by the protagonist.
There is the question of what the clown represents (and occultists could fill volumes with it, there's a reason 'the fool' is the zero card in the tarot). The clown holds a mirror up to our reality (usually our social reality) and exacerbates the absurdities, this makes a killer clown scarier than a "regular" killer, since he's trying to be scarier (in every way). There is a book
The History of Clowns for Beginners that presents/constructs a mythic linage for the clown that includes shaman and trickster gods (as well as a lot of phallic imagery) that may offer a good starting point for a folk horror reading of the clown.
A few other loose ends related to this.
There's been an interesting narrative forming in the media discussion of this about the "reality" of clowns, that these clowns aren't "real" clowns and I just came across this
“The fascination with clowns is really the fact that they’re not real,” says Scott Bonn, a criminologist and professor of sociology at Drew University in New Jersey. “We don’t know what’s beneath that makeup. It could be anyone or anything. They’re actually very frightening.”
Is this a "moral panic"? What is the connection between folk horror and the phenomenon of "moral panic"? The "satanic panic" of the 80s is interesting in this regard, in that it has the characteristics of a witch hunt (the targeting of marginal members of the community, rather than objects of consumer media like the "video nasties" or videogame violence moral panics), and that it (arguably) grew from seeds planted by the mass media with the success of things like 'the exorcist' and 'rosemary's baby' and the corresponding upswing in the belief in satan amongst the general public (along with ex-hippies taking up fundamentalist Christianity in their search for meaning (and there's another thing here about the prosperity of a society and how it correlates with increased cult activity)).
Anyway I read in a thread about police cracking down on people dressing up as clowns the idea of a flash mob, where half the participants would dress as clowns and half as cops and they'd chase each other around...
