A "Hey, Mabel!" is a kind of media story (the term originated in tabloid newspaper offices, but hey, things change) that I guarantee you're infinitely familiar with.
Yeah, it's a noun.
Also called a "Hey, Martha!", a "Hey, Mabel!" is any kind of story that's interesting or just plain odd enough to have the reader, usually personified at a sixty-year-old jewish woman for some reason, shout across the hall to her fictitious compatriot, Mabel, so that she can share in the wacky. It's a tough thing to describe, but the image is pretty striking once you've got it. I like to imagine her as being covered in flour and holding a rolling pin while Mabel's just finished putting the curlers in her hair, but that could be because I've been watching a ton of Lucy reruns lately.
The term really refers to anything interesting enough for a person to pass onto another person, but recently has come to describe things that're just weird - in practice, the easiest way to make (or spot) a "Hey, Mabel!" is to count the number of wacky details in a story - three or more and it'll work just fine. For instance:
Man Parachutes out of a plane. Blah.
Man Parachutes out of a plane Naked. hmm?
Man Parachutes out of a plane Naked with his Golden Retriever. THERE ya go. Mabel's on her way.
It's also worth noting that the term originated as an adjective, ie "a Hey Mabel story," but it's been used consistently enough for it to jump the gap into noun territory.