Answer:
It is harder to mock a person who you can easily see is injured, as it inflicts empathy. However, in today society, we have made a huge joke about those with mental disabilities, or just a low level of intelligence, so in our collective heads, we believe that people we deem as 'stupid' are to be made fun of. After all, they don't understand that they're being made of, now are they? We humans also live the belief that we control how smart we are. If you have low intellect, that is deemed as 'your' fault. Not being born with arm or legs, that, you have no control over, and other people recognize this. Thus, people like this are not as mocked as much as 'stupid' people, but still are. Sadly, many humans today thrive off putting down others, to lift their own selves up.
TLDR: Being dumb is your own fault, so we can make fun of you, being born without limbs is not, so we won't, however some people may still do it, just to feel better about themselves.
(Hope this helps)