Name calling is worse than other forms of lashon hara like telling others about sins a person did. For example, calling someone stupid. People will listen to the name caller it seems to be human nature. But when a person talks about the actions someone else did the people listening make up excuses for what they heard or don’t believe that is really what happened even if it is true, but maybe it was taken out of context.
The Chofetz Chaim does say there is one instance where name calling is allowed and even a mitzvah! When two people are in a fight over what someone said or did to the other and you go over to the one it was done to and say, ‘He wasn’t acting malicious he is just stupid.’ If that will ease the tension and dissipate the fight, then it’s a tremendous mitzvah!