YouTube comments are some of the trolliest on the web, but you don't need an extra add-on to block them — just use Adblock Plus.
Reader isaaclyman explains:
I don't know about you guys, but as soon as it occurred to me that there was an extension to hide all YouTube comments, I found it and installed it as soon as I could.
It doesn't work on the new YouTube layout, though, which frustrates me. Luckily, I just remembered one of the most useful features of Adblock Plus: you can block anything (it doesn't have to be an ad).
So I right clicked on the comment area on a YouTube video, clicked "Block this ad," and moved the slider until all comments had disappeared. Problem solved.
On Adblock Plus for Chrome, just head to YouTube, click on the Adblock icon in your navigation bar, and hit "Easy Create Filter". Then, just click on one of the comments and it should highlight all of them. After hitting "Add", you should be free of YouTube comments forever.