Plenty to admire here, like the lack of judgment towards Minnie's sexuality and Bel Powley's wonderful performance. But the film felt oddly weightless to me, considering how much happens plot-wise.
After thinking it over, I think I realized what the problem was: The Diary of a Teenage Girl respects its characters' motivations, by not exploring them at all. Why is Minnie sleeping with a man twice her age? Why is Monroe, a grown man, sleeping with a fifteen-year-old? The film normalizes the situation, at the expense of actually exploring it. Its idea of empathy towards its characters is simply blankness.
I think Fish Tank tackles similar subject matter far more successfully because Andrea Arnold does explore why her teen heroine is drawn to her mother's boyfriend and vice versa. And when we gain this deeper understanding of the characters and their complex motivations, we begin to actually empathize with them.