There is one good thing about "racist" and "streotyping" jokes, if you're willing to see it. That is: Making light of ones background(others or your own), ethnicity, the way one walks, of ones fashion sense, etc, can serve to belittle the fact someone is different than others and so help to come over prejudices as the joking alleviates fear of abnormality and so opens up geniuine conversation.
Mocking, however is not joking. It's a deliberate attack and an insidious way of masking a jab at someone a harmless joke. That is what is so frowned upon(and rightly so).
Now, most do-gooders(I'm gonna get stabbed for that one) conclude that shouting(more eloquently than this): "THESE PEOPLE ARE DIFFERENT BUT THEY SHOULD BE TREATED LIKE ANY OTHERS" helps the people they are speaking for, when in reality they are urging people to treat them specially(often unwittingly), because of their apparent external qualities(skin/eye color, customs, habits and so on) and not offend them. This however creates a "them" label; the very thing the do-gooders tried to abolish.
So by not making a huge scene every time someone makes light of some percieved trait or stereotype, can actually help more that shouting real hard that the person in question is a filthy racist dungbag and should be castrated.
Of course all this is useless if we(humans) can't learn to be individuals in a group and acknowledging that others are too.
Rant mode off.