The rules here are allowing any criticism, but forbidding any offends, so, I just can give you a tip:
The good criticism is that explains why level is great or sucks, for example: "This level sucks because of: - lot of mistakes in gameplay, chracter can stuck in level because you forgot to make the escape ways, worst graphics quality, too flat scene, that you must fix".
Any reason-less criticism may be offensive. You may to explain same as "You very bad artist and drawing like chicken!", or as "I found mistakes in your art, please fix that". It's approximately, but you must understand how to explain that creation is not good and to don't offend creator for that.