Thumbs-down emoji should do it. Copy Dluvian's reaction to my reply :p I do like Amethyst's freeform unicode reactions...
they are not as flexible, you have to preconfigure them, but nostrudel also has unicode emojis... i see that there is gif and jpg/png ones too, not sure which clients are making those? primal?
Unicode is pretty compact, and carries a lot more nuance than up/down. IDK which clients do image file emoticons, but it seems to me like an inefficient solution to a solved problem...