I don't think it's necessary. You can just add a poll to get a overview if people like certain ideas.
If you really want a dislike button then we could just call it "disagree" button or the poop emoji would be cool too. Dislike has such a negative annotation.
But I agree with Crim. People would possibly abuse it. The like button had the same problem.
OOoh okay it'll be for banned members only so we can dislike every suggestion we see (:
The amount of abuse would be ridiculous
Great ideamaybe not a dislike, but definitely add a button