In order to learn and make things better you _have to_ be critical. But the way things are communicated is very important so everyone is on board.
"We could do better here" - no matter who exactly was responsible in the past.
"This made sense at the time but with what we learned..." - remind each other that improvement and learning is part of the whole deal.
"I like the simple and expressive core idea of this, but if we expand this further..." - elevate and develop the good stuff that's already there.
I make jokes about my mistakes, bring them up early and often. Everything is a bit lighter and easier with a bit of humor and without the fear of making mistakes.
And vice versa it is just as detrimental to be afraid to bring mistakes and inadequacies up and criticize them. It's much more fun and productive if things are continuously improving.