I'm pretty sure this is an instance thing. It's not just karma, it's the ability to see downvoting. So if you don't like the current state of affairs, you need to talk to your instance admins or change instances to one that has downvoting enabled.
Personally, I do not want this feature enabled universally or somehow forced by the app; my instance doesn't have it, and I like it that way. If I don't like something, I can either engage with words or ignore/block/report the person or post. Being able to downvote is lazy engagement, and it doesn't actually do anything beneficial to further the discourse.