4 years old being mean

Our 4 years old seems to not be aware that some of the words may hurt other’s feeling. And no matter how often and hard we try to explain to her that calling names is not fun, she does not seem to understand why. She started kindergarten recently and it all started since then. I knew she would bring some words and behaviors, but it looks unusual and disturbing for me that she does not understand words are connected with emotions. She will call “ugly”, “shit” and other mean words to her brother and will laugh about it. When we ask her how would she feel if she was called that way, she will say she would find it funny.

My question is, is this normal behavior for 4,5 years old? Should she not be aware of what words do by this age? Should mean words be fun for her? And how to explain to her it’s not fun in a way that she actually understands?