I know, I know, crazy idea right here.
However, I don't think its addition has any practical use on itch.io.
On things like Q&A websites, they are useful to gauge which response is best, and is visited by people who are inclined to share real knowledge and thus vote very legitimately while doing so. Note, voting is perceived as a near objective quality of somethings correctness on these websites, which is why it is used.
However, useful itch.io comments are all game feedback and criticism, while the site is mostly visited by people who are inclined to play the game rather than build on to it. Game development has no "correct answers", so allowing people to vote on things they might not like for personal reasons and having them feel like objectively bad ideas is hurtful to the idea sharing space. What we need is constructive progress, not destructive progress.
People already have the ability to reply to comments and give their own feedback on why their response may be good or bad, which; is far more productive, allows the developer to know more about the issue, and also gives commenters trying to give constructive feedback a chance to elaborate on their point. Replying to comments with a description of your feedback is the ultimate filter to people who don't actually care about the game, as they won't bother to put themselves out there if they themselves have to be prone to criticism as well.
Overall, upvotes and downvotes make feedback discussions feel more like a court wherein you're trying to appease a faceless group of people who don't have to say anything besides agreeing or disagreeing completely with you. This is detrimental, as feedback on things you love should feel more like a room full of enthusiastic people sharing their ideas equally instead of anonymous judgement where you are punished for putting yourself out there and making yourself prone to criticism.