If I’m doing the review, I try to find at least one or two items to call out as great ideas/moves. Even if it’s as simple as refactoring a minor pain point.
If I’m being reviewed I always make sure to thank/compliment comments that either suggest something I genuinely didn’t consider or catch a dumb move that isn’t wrong but would be a minor pain point in the future.
As you note, code reviews can be largely “negative feedback” systems, and I find encouraging even a small amount of positivity in the process keeps it from becoming soul sucking
So actually putting positive comments in the code review isn’t really much appreciated.
I gained this habit and now for me, a comment is a suggestion of improvement, I deliver praise out-of-band.
It's a horrible practice with adverse incentives, and one of the reasons I'm glad I no longer work there
(and easily gameable, anyways - people would just DM each other patches they were unsure of, before submitting an actual CR)