If this instance was all about math and someone claims that 1+1 = 11, then I can see how being able to downvote them would be a useful feature. But for people posting nudes and porn? I don’t really see the point.
If large numbers of people don’t want to see something in a particular community, perhaps the poster should consider posting it elsewhere or not at all.
I don’t think there’s anything wrong with using them as like/dislike buttons on LemmyNSFW. This isn’t a “does it contribute to the conversation” instance for the most part (this particular community being an exception - I upvoted this post because it’s worth discussing even if I disagree with OP’s request)
Porn is a subjective art, and some is better or worse as judged by a given audience. I want the things people like to rank higher than the things they are meh about, and the things they actively dislike to rank even lower than that.
As for the difference, a downvote is a much stronger statement. On the rare occasions that I do downvote here it means I truly want to see less like that for whatever reason, while posts I just don’t care about I don’t vote on, and I think that’s an important distinction to have available. (There are so many times I’ve wished I could “dislike” Twitter posts…)