The old adage that "sticks and stones will break my bones, but names will never hurt me" applies to everyone, no matter who or what they are. It doesn't matter what people call Kim Jong Un, since he's North Korea's ruler. If he can't take being called simple names, he should step down.
The Chinese should not stop calling Kim Jong Un fat. Why, you ask? Because, let's face it, Kim Jong Un is fat. Moreso, it is actually pretty funny how China is needling the North Korean leader. It must drive him nuts be be disrespected like this. Entertainment at its finest.
In no way shape or form do I believe that the answer to the many problems posed by North Korea lies in other nations and peoples moderating the way in which they speak about the country or its dictator of a leader. They should not feel the need to walk on political eggshells around North Korea.
No, the Chinese should not stop calling Kim Jong Un fat, because he is. When a person decides to hold a position of public power, they must know that they are going to be the subject of scrutiny. This is especially true for someone who decides to be a dictator. He should learn to dodge a few verbal arrows.