I think if it's judiciously used by viewers it's a useful thing, like when someone has a clickbait title or picture and the video is nothing about what it claims to be, it deserves a dislike. Also if something is an obvious ripoff.
One problem with the dislike is that sometimes people don't seem to understand what it's for. Like, if someone is giving tragic news in a video, that doesn't mean you should dislike the video. I think sometimes people don't want to "like" tragic news, understandably, but that's not what the dislike button is for.