I see karma on posts fluctuating (in particular going down) more than I would expect coming from other vote-based websites. Is downvoting really used here for posts that are not spam or trolling? Or do people just change their minds a lot?
The FAQ has: We encourage people to vote such that upvote means “I want to see more of this” and downvote means “I want to see less of this.” But I guess I’m surprised if people actually behave that way? And that some posts are controversial enough to receive active downvotes vs passive ignoring.
Speaking for myself, when I consider what post to curate, I let my attention naturally go to the top 5 or so recent high karma posts, as well as to the posts that other mods nominated (we have a mod-only UI that shows all curation nominations). I also ask myself what posts I liked lately, and occasionally when I read a post I immediately think "wow this was excellent, I bet I'll want to curate this 5 days from now" and nominate it. For example, this happened with a post recently, where I wrote a comment about why I liked the post as soon as I read it, and then still endorsed it 4 days later and curated it.
Overall karma plays an important role in what posts I consider. But to answer your specific question, about whether a higher karma score is something I consider a positive signal, the answer is no.
My rough internal question is "Was this idea important/interesting/useful enough, and was it written clearly/concisely/enjoyably enough?" and don't care about the karma. That generally produces a binary "yes/no".
The main way I use karma is to second-guess myself. If I think a post should be curated, but it only got like 35 karma, then I will spend some time considering the hypothesis that "This post is really well aimed at Ben in particular, and actually a lot of people won't be that interested to read this."