To be clear, Jack and Diane has nothing whatsoever to do with John Cougar Mellencamp's 1982 song of the same name. That song was a brilliant piece of Americana about two kids, a boy and a girl, growing up in Middle America. This movie is a disorganized piece of indie "realism" about two girls in New York who approach falling in love with one another. The problem is that the screenplay doesn't have enough confidence in its characters to develop them in a way that makes us care.
The major focus of the film is Diane (newcomer Juno Temple), a wide eyed Brit with a mop of blonde hair who chooses baby doll dresses to be her ever-present wardrobe. She's a cute girl who misbehaves, drinks to the point of vomiting and has frequent nosebleeds. The other girl is Jack (played by Riley Keough, the granddaughter of Elvis) a skate-boarding butch lesbian, a loner who finds something to like about the dysfunctional Diane.
The girls meet in a store one day and have a few shy, awkward exchanges before the moment is interrupted when Diane has another nosebleed. The relationship is mostly physical. We see them kissing, there are a few hints that they are thinking about sex, but the movie never really gets them there. What passes for romance is just a few scenes in which a mutual attraction. An attraction of personalities, however, never materializes because the movie doesn't really deal with emotions or teenage awkwardness.
There is something in these two girls that, in a better movie, might have made for a pure down-to-earth love story. The problem is that the screenplay won't have it. It keeps intercutting their budding relationship with a lot of episodic nonsense, like a silly scene in which Diane interrupts a date with Jack so that she can escape into the bathroom to shave her pubic area, or an early scene in which Jack is hit by a car which is never mentioned again.
There are lots of camera tricks and clever editing that take away from the underwritten characters, like the director's odd visual effect of a reoccurring microscopic image of something monstrous that is growing inside of Diane. It isn't clear what the manifestation is, maybe we're suppose to think that it is a physical rendering of her twisted feelings about Jack. We see internal organs with hair slithering tightly around them, but we are left to assume what that might be. We are led to believe that it is the manifestation of this new lifestyle but you're never really sure. The scenes are disgusting and fall on the story like a ton of bricks.
Those scenes seem to indicate that the filmmakers didn't know how to create characters with genuine emotions. The actresses playing the girls have screen presence but what they have to talk about is dull and uninteresting when it isn't being intruded upon by another dramatic element. The movie moves away from their relationship as an effort to keep from having to really deal with them. This is a very confused movie that leaves you scratching you're head when it's over.