In this Seinfeld scene, Kramer is offended that a record reseller does not know the greatness of Sergio Mendes. It is not clear whether the writers intended for Kramer himself to be part of the joke; as important as Sergio Mendes is for bringing Brazilian music to the United States, he has not been very well known in Brazil, as he was exiled throughout almost all of the military period of 1964 to 1985, but became famous among sophisticated listeners in the U.S.
I learned of the U.S. band Black-Eyed Peas when I was walking through a book store with my mother. I noticed a display of CDs featuring their work with Mendes -- I had no idea who they were, but bought a copy without hesitation. The best known track is Más Que Nada.
The Mendes story is placed into context in the Netflix series Break It All.