Yes, 13 Reasons Why can be considered by some like a depressing series. The first season showed different rape scenes, and this is already too much to grasp for some people, especially those who can relate to what Hannah Baker went through. It can make people relive the same experience all over again.
It also showed Hannah doing some self-harm and making a suicide attempt. After the series was shown, there were more suicide attempts among those who watched the show.
They started to feel that it is okay to have suicide mainly because they have their own reasons for taking their own lives. Those who are already depressed are not recommended to watch the series.
Oh yeah, the 13 Reasons Why is a really depressing TV series. Just the first season itself contains several scenes with graphic rape and quite a number of suicide attempts. This actually prompted discussions on mental health and sexual assault for some time. The movie is shot in such a way that every moment is in a scene is covered.
For example, in season 2, when Tyler was being raped, well, the brutal moments in that scene do not need to be shown before I believe that she was raped. The scene is such a brutal one, which I do not necessarily need to see before I empathize with Tyler.
Viewing the case of Hannah Baker also, who suffered slut-shaming, bullying, and physical assault, when she met Mr. Porter makes her come into a complete depression. This leads to a mental depression for her, and how she killed herself is shown in the last episode of season one. Hannah cuts her own wrist, and she bleeds out till death in that frightful scene.