There's a weird schism between THE MUNSTERS and their accepted rivals, THE ADDAMS FAMILY. From a distance they both look about the same: B&W sitcoms about ghoulish maniacs that haunted the American airwaves during the 1960s. They're the same, right?
Not at all. When you dissect THE MUNSTERS, what you have left is LEAVE IT TO BEAVER in Universal Monsters drag. They might look like outsiders, but it's less idiosyncratic than a Halloween episode of THE DICK VAN DYKE SHOW. If you can't tell, I'm firmly in the Addams Family camp.
Which is why, despite the Jack Pierce make-up and gothic accoutrements, it's not a huge leap that General Mills would ask THE MUNSTERS to help sell their inedible, newspaper-flavored cereal Cheerios. If Lily Munster gave you a bowl of it, you wouldn't have much reason to suspect that it held anything other than cereal, milk, and maybe a few stray bat hairs. I doubt you'd be quite as open to the idea of eating something Gomez or Morticia prepared.
Below is the vintage Cheerios ad in question, starring Herman, Lily and Eddie Munster.