"This is great, this is ok, this is great." is an amazing tool. People will always focus on the negative, so presenting a negative as a positive, but juxtaposed with even better positives, shifts the scale. It makes people think about it, without being too hard on themselves. I call it the compliment sandwich.

Another tip that has served me well is the tick of approval, followed by the question "With this part, what would you do differently next time?". It frames it as self improvement feedback.

That being said, I'm not part of a traditional code review process, so this could all be totally misguided advice.