Reactions are single character emojis or other character, and later enhanced with support for shortcode for custom images

Some clients use "+" as a default reaction, and render it as either ❤️ or 🤙

Reactions drew the ire of many when some clients erroneously would notify/buzz users when they were mentioned in a note, as creating a reaction was considered low effort.

Per the spec, a dislike is a "-" as the reaction. And a like is every other character or combination.  This disparity means tallying for likes vs dislikes is a fools errand as dislikes require significantly more effort for the clients that even support it.