Yeh probably not the best use of -karma, unless it's shitposting which is loosely defined on this forum as "posts with few words" as opposed to poor quality posts.
If you gave -1 to a user for a post you considered crap that's not going to be an issue overall. We're not reviewing every -1 here, only targeted -/+ karma towards a specific user, from a specific user. Negative karma should otherwise be for disruptive posts at a minimum, such as those intended to game the system or breaking forum rules, not just poor quality posts.
If in doubt don't smite others, that's my motto 
I think, using negative karma this way might not be ideal unless it's for something like shit post, which on this forum is loosely defined as 'short posts,' rather than low-quality ones.
If someone gave a -1 because thought someone's post was bad, it’s not a huge deal. Will not scrutinise every -1; only look into patterns of targeted karma, whether it's positive or negative, between specific users. Generally, negative karma should be reserved for posts that are disruptive—like trying to game the system or breaking forum rules—rather than just low-quality content.
When in doubt, it’s better not to downvote others.
Think of this type of post as a reply, so when you come across several of these in a row, it's a bit tiring. OK, maybe I oversimplified things here, but I hope that what I'm trying to explain is understandable.
Here, it would be useful to see which post received negative (or positive) karma so that the person who wrote the post would recognize the poor quality of his post.