Is this one of the most acceptable forms of discrimination?

Yes, unfortunately some people think that discrimination or prejudice towards certain groups is somehow acceptable. People who are rather unintelligent are among those who get made fun of or treated rudely and that is considered socially acceptable. Another such group is people who are overweight and yet another group is people who are different from the majority in some ways, through no fault of their own. It is stupid that people think it is acceptable to treat some people badly or make fun of them for weaknesses they can't help having.