ZERO possibility of dispute that it is an obscene or "dirty" joke  
              
 ZERO dispute that critics have CALLED it racist, or that, in addition to its primary attributes, some people could see it as racist, and that if the whole incident is notable, so is this.      

 To state in the article that it IS racist is to imply that WIKIPEDIA certifies as factual that it IS racist, and that is POV.  

 The joke, like all "'yo mama" jokes insults the MOTHER, and thereby, OBLIQUELY, the progeny. In this case, can be held NOT to violate BLP, since Obama's mother is no longer "L". 

 Again, doesn't say Obamas father was a dog, it specifically says his father was NOT a dog, just that he should be glad that his father ended up being his ACTUAL biological father, because, given the alleged sexual habits of the mother, it could have been anyone (ie including the dog - point of the joke is in essence that Obama's mother slept around).   

 There. A structural and grammatical analysis of a 'yo mama joke. The things I do for Wikipedia.