It has just occurred to me that this is the manga equivalent of Jane Austen's Northanger Abbey. That book was a comedy all about mocking "gothic novels"--the popular kinda-horror novels of the day that had lots of dramaaatic scariness. So at one point the heroine, who has gone to stay in a dark old mansion that seems really creepy and is investigating her mysterious-seeming bedroom with a big, ebony chest of drawers finds a mysterious-seeming piece of paper with writing on it. It's at night, she's scared, and just as she finds that slip of paper the thunderstorm reaches its peak, there's a massive clap of thunder, she faints dramatically . . . and the next morning finds out it was a laundry list.
This is kind of doing that; it's bathos. The comedy point isn't the fart joke itself, the point is deflating the expectations of dramatic vampire stuff--and what does that more completely than a fart joke?