Is making fun of fat people bad?

Ok so I was at school and the night before school we had a 10 second earth quake. And so everyone was talking about it at school the next day and if they felt it. And so everyone starts asking me what i thought of it. And i told them this fat girl in our school must have fallen down the stairs in the middle of the night to get something to eat. Is that mean? Why? Technically its the fat persons fault that there fat. Just wondering