It's usually obvious when someone is posting to express their honest opinion and when they're doing it to be deliberately abusive, offensive or disruptive. It's not difficult to state your opinions in a positive, constructive manner, even when it's something you don't like. 

For example, stating something is ugly without giving any reasons why you think so is not constructive and just leads to more negativity. Instead you could say something to the effect of "This render could be a lot better if..." and filling in with ways you think would improve the image. There are different people here with varying levels of skill and a lot of them post their work looking for feedback on ways to improve.

The goal is not to sensor anyone's opinion but to foster a more friendly and productive environment for everybody.