I must preface everything I'm about to say with an admission: I personally am awful at trying to imitate foreign accents. When I try to do a British accent, I sound like Jonah Hill in that Forgetting Sarah Marshall scene where he does an offensively bad imitation of Russell Brand ("I feeel terriboo!"). Fortunately, I don't get paid to sound like someone from another country, so it's no big deal when my attempts at accents are cringe-worthy. Unfortunately, actors in feature films can't say the same thing. That's why it's a little bothersome to me that the British Cara Delevingne's American accent in Paper Towns is so bad.
Major motion pictures have access to the best dialect coaches in the world, so I find it puzzling when stars sounds noticeably different from the nationality of the character they're supposed to be portraying. Feature films are meant to be seamless. That's why they have 16-hour shoot days and disgustingly huge budgets; every little detail is accounted for down to the color of the background actors' socks. In respect to characters' voices, this seamlessness is best represented by those moments where you see an interview with an actor and say "Oh my God, I had no idea they were [blank nationality] because they sound so [other blank nationality] in their movies!" Viewers are used to these accent chameleons and such a level of meticulousness in movies that it's very noticeable when an actor doesn't perfectly grasp the dialect of their character. It's like decorating a house down to the salt and pepper shakers but failing to put on a roof. It just doesn't work.
Some people have argued that Delevingne's accent is acceptable because what qualifies as an "American accent" sounds very different depending what part of the country you're in. This is 100% accurate, but the thing is that Delevingne's Margo doesn't sound like the way that ANY particular region of the nation speaks. I would argue that if you were talking to her in a bar, after about one drink you would lose the ability to bite your tongue and would have to ask "Um, where are you from?" I would also wager that your first guess would not be New York, nor Charleston, nor San Diego, nor Wichita, and especially not Orlando, where Margo lives.
Still, Delevingne's American accent is far from the worst used by British actors in film. Remember Emma Watson in Perks of Being a Wallflower, or Gerard Butler in The Bounty Hunter? Comparatively, the supermodel's voice work is not nearly as distracting. And if you were in a bar setting with Delevingne, you actually might get a better version of her Margo accent: she joked with E! that she's sure her accent is "better when I have a few drinks." Also, she did really try to vocally prepare for the role; the model spoke in her "American accent" in her day-to-day life during the filming period, even when she was doing interviews.
The verdict? Delevingne likely won't be getting any elocution awards any time soon, but that doesn't mean she didn't put effort into her portrayal of Margo. Even if the accented American English is distracting to some viewers (i.e., me), Paper Towns is a beautiful film. My best advice? Just watch the movie and try not to overthink the accents. If Paper Towns is a house without a roof, well, at least it's not raining.