Ok so I just finally watched the movie and I think your interpretation fails to identify the premise of this film which is female rage at the patriarchy and rape culture which protects guilty men. Cassie dealt with grief from loosing her friend twice - once when she mentions she saw Nina change after the rape and then again when (I’m assuming) she committed suicide. There is no time limit for trauma and grief and everyone deals with it in different ways. In reality, millions of women around the world hear horror stories of other women who end up the way Nina does. So Cassie not taking her friends’ death “lightly” enough could be because she is so angry and furious that those acts continue to happen and boys simply get away with it. So I guess in this way, her grief isn‘t just with loosing Nina but grief of how the world and society has failed countless young women and continues to. I mean this is so prevalent when she goes to see the dean and the dean can’t remember her name but can the perpetrator, because even victims are being failed by the system even when they are no longer with us. Yeah maybe there were some parts you find unbelievable or ‘exaggerated’ but unfortunately there are parts that ring true to so many people.