Your profession doesn't make your opinion any more valid than anyone else's, or less valid.
There is a point in being nice, it's part of being sociable. We could have all the best solutions in the world, but if we can't share them in an agreeable way then they are going to be useless
Think of a world where parents mocked their toddlers for presenting them with a anotomically and structurally defective picture of their family and home. Would the child continue to create with happiness or just associate experimentation with negative response?
I'm comparing the act of creating and reviewing. More explicitly I wanted you to think about how a new designer would respond to overly harsh and non-constructive critisim in the early stages of developing their skillset.
There's no point being nice to people when they present flawed work - that just propagates bad work.
(and my CTO is way harsher on my Pull Requests than anyone has been on the Wiki redesign anyway)