George and Jerry are native New Yorkers, from Queens/LI. Kramer & Newman probably are too (at least Newman, given his accent).
George works for the Yankees at times and in his interview with George Steinbrenner, calls them "our beloved Yankees". Mr. Costanza berates Steinbrenner about trading Jay Buener. Jerry and George are thrilled to meet ex-Met Keith Hernandez. References are made throughout the series about watching or taping ballgames.
So, are George and Jerry Mets fans or Yankees fans? Any true native New Yorker (and I am one) will tell you you can't be both.