Biography
A character, often a Cloudcuckoolander, whose innocent attempt to make something (typically a drawing) comes across as Nightmare Fuel to the other characters. Unlike with normal Nightmare Fuel, other characters in the show will notice how horrifying the drawing/poem/song/story is, but the innocent NFSA sees nothing wrong with their tale of baby-eating zombies or the drawing of a "miniature poodle" that looks more like the ungodly offspring of a hellhound and a C.H.U.D.
More generally, this kind of character will be simply a casual observer to some frightening or grotesque phenomenon who either doesn't bat an eye or responds with some kind of snarky comment (and no, they're never meant to be a Stepford Snarker). It will often be a child or a "simple" person who doesn't comprehend the danger, or else a Jerkass or Misanthrope Supreme who genuinely doesn't give a damn what happens. Either way, you can expect that saner characters will be almost as creeped out by this character's reaction as by the actual threat.
Frequent features of the works of a Nightmare Fuel Station Attendant are:
A childish outlook on mature issues, such as a stick figure drawing of a bloody massacre.
Disturbing and mundane elements mixed, often in the Bread, Eggs, Milk, Squick or Arson, Murder, and Jaywalking forms.
Lack of knowledge of proportion, perspective, shading, etc. produces monstrous looking drawings of real objects, or images which just look wrong.
Fridge Horror, especially when a sentence is a Double Entendre or could be looked at in different ways. For example, while being alone in a dark abandoned school is scary, not being alone is possibly even scarier.
Nothing Is Scarier in general.
Frequently combined with Room Full of Crazy, Creepy Child, or Dissonant Serenity tropes when the writers need to give their characters that extra little bit of creepy. Sometimes, this trope is subverted, turning this trope into Nightmare Retardant Station Attendant.
Compare Nightmare Fetishist, Nightmare Fuel Coloring Book. The very existence of the Unintentional Uncanny Valley proves this trope is Truth in Television. Not to Be Confused with Creepy Gas-Station Attendant (not that those can't be this).