You know what, stereotypes are damn funny, and so is making fun of celebrities. The difference here is that one type of humor jabs at an individual, while the other just panders to the most basic, simplest choices of social signifiers, like skin color or eyelids for a quick laugh. Never mind diet.
Then there’s the issue of authority and license, and proprietary humor, and all that jazz. We still play up stereotypes. They’re just delivered by people deemed acceptable to kid around with the material, those we’ve entrusted to re-appropriate the humor, striping much of the tension away so everyone can laugh. Thank god one of them is Fred Armisen, the Portlandia guy who does a wicked Obama impression on SNL.