One of the purposes of voting/liking is to provide a quick way to engage a post without the full steps of writing a comment.
Having a like/up-vote button reduces the amount of "nice!" comments and such, and lets the comments be focused on making a point.
But if you don't have a dislike/down-vote button, then people who disagree/dislike and are feeling the need to engage have no other recourse except to leave a comment, (or ignore it). And if you have options for a quick positive response but no options for a quick negative response, then the comments can become majority negative because there's no other outlet for a negative response.
I absolutely don't think down-voted comments/posts should be hidden or anything. It's just nice to know as a reader how others feel about it, and it gives the reader additional sorting/filtering options.