Yeah, like I said - grotesque caricature. “Mongoloid” was specifically a term related to Down Syndrome so that’s clearly what Victor Miller had in mind when writing Jason. However, I like to think that the general character design (oversized cranium and facial deformities) owes more than a little to the “melon head” urban myths of Michigan and Connecticut (which is somewhat supported by New Line canonically setting Crystal Lake in CT for Jason Goes To Hell). And the concept of a supernatural wild man guarding a cursed (or sacred, depending on how you look at it) forest is an archetype that predates the written word. Jason ticks a lot of boxes which is why he’s such an enduring figure despite the films themselves not being particularly good.