I think that the film ultimately tries to be something that it's not. The message at the end of the film is one fairly typical of most films featuring a teenage character, "Don't be stupid, you'll regret it," however the issue here is that most of the film is spent glorifying the stupid things that the protagonist is doing like drugs and sex.
The acting is really good. Bel Powley delivers a somewhat believable experience, making the audience believe that she is really in love with this man, however it is also obvious that she is too old to be playing a teenager. Kristen Wiig really shines in her supporting role, no matter how small it is. She perfectly portrays both the bad and good mother that the character is.
In my opinion, "The Diary of a Teenage Girl" had a lot of potential, but it just tried to hard to be something bigger than it is, dragging on for too long despite its runtime at only an hour and forty minutes.
The Diary of a Teenage Girl
Big Tuna's Rating: B-
How Did I Watch It?: Redbox rental.
Had I Seen It Before?: No.
Would I Watch It Again?: No.