PieFed v1.4 (to be released very soon) will have emoji reactions on posts and comments. An emoji reaction can be a unicode character like ๐ or a colon-delineated string like :piefed: which gets automatically converted into an image when the post/comment is displayed.
Setting a reaction uses the same API endpoint as casting a vote (and behind the scenes reactions are federated as Likes, for Lemmy compatibility) but with an emoji element added to the JSON payload.
Also when looking at a post or comment in the API there is an emoji_reactions element that can be used to get a summary of the reactions that have happened so far. Compare https://crust.piefed.social/api/alpha/comment?id=456791 with https://crust.piefed.social/post/84566#comment_456791 to see what I mean.
Do a GET on /api/v3/site to retrieve a list of all the custom emoji (the ones with :something:) that this instance knows about and to find what images to convert usages of them in emoji_reactions to.
Except that reactions count as upvotes, so I don't think it'd actually be a good way to express disapproval without downvoting.
Upvotes aren't really controversial though. One assumes if you give a thumbs up or a :100: that it counts as an upvote in itself, which is fine. But many other reacts may have more abstract uses.