From the title, I thought that you meant you disliked the Discourse API. A few more words in the title would have saved me reading this topic, e.g. “How do I dislike a post using the Discourse API?” As it currently is, I would have ‘disliked’ your post.
There are no dislikes, down-votes or anything like that unless you add that feature yourself. There are lots of topics on this including this one: