People in powerful positions, celebrities and thin people are admired, generally speaking. We want to be like them, have power or fame or (in U.S. culture) look thin. We would not like to be disabled or overweight. So, jokes about the former don’t hurt as much, while jokes about the latter are considered mean. Making fun of people is rude. If you want to amuse people with your wit, try using situations as your subjects instead of people.