No, Seinfled is not racist, because he is simply making personal jokes based on his own experiences. In our society, we cannot ignore that people have differences. Seinfeld did not stereotype people based on racial characteristics or other arbitrary things. Rather, he was just making jokes about things that he experienced in life.
All those talking about the Puerto Rican Day episode are full of s--t and hypocrites. I'm Hispanic myself and it wasn't those type of people who were racist against my kind (not that I care, cause I'm not one of those Al Sharpton types who scream racism at everything). The people of "Friends" -- those type of people are the real freaking racists with their puritan attitudes. But anyways, "Seinfeld" is hilarious.