Even Paul Graham thinks voting rings are a problem, to the extent that he said (somewhere, I can't remember where) that a substantial part of the HN codebase is dedicated to measures against spam, voting rings, and other abuses. Steps to prevent voting rings were among the top measures he deemed necessary.

A dislike mob is just like a voting ring.

Another dimension of the problem is the scenario where a user takes a dislike to someone else's comment, and goes through that other person's comment history, downvoting every one. That's obviously an abuse of the voting system and it makes sense to organize things so that it is minimized. (One way to do it is to set a time limit on voting on a given comment.)