I've seen this done on other forums and imo it makes them feel more engaging, both when posting and when reading - the ability to add a catmoji to a post to signify that you've read and liked it without making a reply post.
A lot of the time I hesitate to reply when all I have to say is "thanks!" or "agreed" etc because it feels kind of spammy, but not replying at all seems like I'm not letting the poster know I liked it.
It could possibly be difficult moderation wise and/or cause clutter to allow all catmojis as post reactions (I'm not sure, just pondering), so another option would just be to have either a single one that functions a bit like a "like" on facebook/twitter/etc, or a small set of them. It may also be worth considering having only the poster be notified when their post is reacted to rather than having it publicly seen, to avoid potential competitive/envy type feelings about some posts getting more reaction catmojis than others if that is a concern.