Downvoting has its uses, and with ssb being so decentralized, downvoting would eventually arise. However I'd recommend framing it as not the opposite of upvoting, and not side by side with liking. Downvoting is more useful for flagging a comment as unnecessary and harmful. So it should look like a report or flag button. If it's symmetrical to 'like', it may look like the use case for disagreeing with an unharmful and valid argument.