The laws of Loshon HaRa apply whether it is the spoken word, written word, or even gestures. While I'm inclined to think it is permitted (and have done so on select occasions) how would you understand the halachic status of a down vote with respect to hilchos loshon hara?
- Anybody can ask a question
- Anybody can answer
- The best answers are voted up and rise to the top
The real answer is that a down-vote could mean "I don't think this is the kind of question mi.yodeya should focus on", without necessarily implying anything bad about the person who asked it.
Being able to respectfully disagree with an opinion -- while maintaining respect for the person who holds it -- is a hallmark of healthy Jewish learning. R' Moshe Feinstein was asked by someone in Bnei Brak who wanted to give a halacha shiur that would quote the late Chazon Ish, but occasionally disagree. R' Moshe wrote that any respectful quoting of the Chazon Ish's views is a respectful thing, even if you then describe why you disagree. And that the Chazon Ish never expected or demanded that everyone agree with him all the time.
But even if a down-vote was construed as deleterious to the asker (which it shouldn't be), let's review the Chofetz Chaim's criteria for Loshon Hara for a creative purpose:
If someone writes something incorrect, it would be proper to downvote him so that other people do not learn to act incorrectly. StackExchange relies on downvoting (not flagging, as some people assume) to mark incorrect information.
Downvoting a wrong answer would be permitted as it has a clear constructive purpose. The Chofetz Chaim and other poskim all permit speaking Lashon Hara for a constructive purpose (subject to several conditions). Helping people learn the correct Halacha and not stray after an incorrect idea is a toelet, a very important one indeed.
Let us also remember that, at least according to the Chofetz Chaim, speaking positively about somebody is also forbidden by the laws of shemirat halashon, so giving up-votes would be equally problematic.