To you maybe its a joke. But to her, its a topic. 
What the fun you got when you making fun of others? 

She is ugly and you making joke from it. You even tell it straight to her face and say she wont had any friends bcos of it. Dont you know you had killed her self-esteem? She is fat and you making fun about it. Saying that she didnt look good in those clothes and keep reminding her that she fat. Congratulations! You had kill her self-esteem, again! She is a quiet person and not really good in communications, so she didnt talk or speak even a word for a long time and when she start to talk, you said (sarcastic way of course), "Oh, you speak. I thoughts you mute." or "Oh, you still exsist. I thought you had been kidnap or something" While she just sitting there next to you from the early. Its just, she didnt feels like want to talk. But when she had the mood, you kill it! 

And when you throw questions, "Why you didnt talk? Why are you like this?" What should she say? How should she answer it? What do you expect her answer will be? So she just remain silent and throw bitter smile. Because she dont know it either! That is herself! That is how she is! So deal with it! Dont like it? Go find someone who willing to talk with you 24/7.

Stop mocking or making fun of others. She had to struggle to love herself and you keep playing with it. Didnt you know how hard it is? To hate your own reflections and attitude? And when she talk back or get angry, you'll go, "Man, that girl cant even take a joke! She crazy sensitive!" Hahahaha. Yes, she is crazy! Her friend even say she had imaginary friend just because she heard voices and noisy sound when fall asleep. While actually its a sleep disorder. Its not just a sleeping problems, its connected to others things which had something to do with her brain. Her conditions is just hard be understand. Without knowing that she struggle lots inside, you mocking and make fun about herself, you called her emotional, sensitive, ugly. Why? Are yourself is perfect enough? I know you skinny, you dress well, you had the face but why dont you have the heart? Please be consider about other feelings. They may not show it but they hold it inside. They hurt inside. 

And hurt from inside is hard to heal. It takes lots of effort and long time just to heal. 
 
"Im sorry its just a joke. Dont be so sensitive. Play along."

If you still think like this, thats mean you still dont understand.

Its not like she cant take a joke. She love jokes so much. But making jokes from others and you expect her to laugh to that joke, what that makes she look like? She laughing because that person mocking herself. She is the materials of that jokes and she laughing about it. Meaning she accepted it! 

Sigh.

You! Be grateful cause you are lucky enough to not experiences this suffering. I mean it.