I think it would be great if you could pin comments, but only in a thread you've made. If you have a thread about something and want to add a message/detail, it would be handy to be able to pin that comment.
Would the post get moved under the original post or would the post just be highlighted, and a button at the top of the page to view the highlighted comment? Feel this would be most beneficial to the Q & A Section, however, and not much use other than shits and giggles on general posts to push a certain narrative the OP agrees with. +1 for Q & A. Neutral to general use