I think there are a few people karma farming. But I think the biggest issues are:
1. People not knowing how to give good feedback. They might not know how to put their thoughts on the game into words, so they just say something fairly generic that captures their feel.
2. People being afraid to upset people by saying something negative. Some people are probably worried about hurting the dev's feelings or coming across as condescending. And this can also be influenced by number 1, where they don't know how to give constructive feedback or word the things they didn't like in a way that is helpful and kind.
3. Just not having any particularly strong impressions that they can put into words. I've had this happen a few times while I was leaving feedback, although I've been trying to leave helpful comments and at least be specific about what I liked, even if I can't think of anything that needs to be improved.
I think something that should be included in the information and resources provided by the hosts of a game jam is information on how to give helpful feedback. That way everyone is on the same page and people who are less skilled at giving feedback -- because it is a skill -- would have some guidance.
Just my thoughts on the matter. :)