I’m not an expert, so those who know more, please correct me.
FYI: with the script User Details on Hover you can also see karma in Lemmy. Example.
From what I understand, there are technical issues with this. Allowing people to hide the karma if they want is easy. But blocking people for seeing karma even if they want is much harder. Note that if you want karma for posts and comments (to be able to sort the most voted ones), then the user karma is just a very easy query away (just sum the karma of that user’s posts and comments). EDIT: I realize that this would not solve the issue either: There are technical ways to do anonymous and auditable voting, but I think that would be too overkill for the fediverse.
IMHO as a random user is that, given the nature of the fediverse, that makes more sense to be an option for instance admins. I’m personally more inclined to leave that decision to each user, but I see how the network effects play a role and how someone would want to enforce their decission on their own instances.
Anyway, it’s an interesting discussion and I like to try to understand the consquences of each implementation.