Ted Heath is reputed to have said that That Was The Week That Was caused the death of deference in British society. It's perhaps questionable from a sociological point of view but a nice soundbite all the same. It also highlights one of the key aspects of political comedy - it's disrespectful. It's hard to be polite to someone while you're making a joke about them.
If polcom (neologism alert) demands disrespect, can it survive in a political environment currently in the grip of an epidemic of deference? American journos started asking themselves just this question when Obama got elected. Some comment boxes were naturally outraged!
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Obama certainly seems unjokeable in some ways because a) it could be or look racist and b) has an aura of sincerity and idealism (with his confident smile and kind eyes) that would make being anyone being disrespectful look like a bit of a spoilsport. For many people in America and internationally 'the man who got rid of Bush' will probably be beyond censure for a while (even if Bush was going anyway).
However, nothing lasts for long.
This is obviously less of an Obama-joke though and more a joke about African Americans, or as some might say 'racist'. Or a joke referencing racism. Take your pick. The wider point is that Obama is an unknown and the only real fodder for comedy at the moment is his appearance. The 'real' Obama jokes can only be made when he starts doing things that the people who voted for him don't like. (Though consorting with evangelicals and the Clinton family has upset at least one person but he's not laughing.)