I think it depends a lot on the situation. I've had stories where I've considered leaving a note because the story itself was great, just not what I prefer, and I know authors can feel worried over dropping viewer count. In cases like that, I believe there is value in bringing up that you are dropping it, as you can add in that you think the work is good. That said, I almost never comment on anything. Everyone should try to consider all the viewpoints involved -- just saying you are no longer a reader serves no purpose. Saying you are dropping it because you dislike where it is going but think it is good quality though? That tells the author that they are not doing anything wrong and that even someone who is no longer interested supports them continuing.
That said, I can also understand commenting that you are no longer going to read when someone decides to delete 90% of their work to publish it, as that feels a lot like betrayal for many people. (This one really bothers me, though I can see the purpose, it still feels wrong to remove things instead of just stop adding new.)