Here's a picture of the logo, I'm trying to find out the one next to the Mets' hat:
The image is from S1E3 of Seinfeld (The Robbery) so it would have to be from around or before 1990.
Here's a gallery with more images of the hat in question: https://imgur.com/a/RPBNvMV
So I'm thinking the logo is most likely a baseball team because the rest of the shelf is baseball themed (there's a glove behind Elaine's head). From left to right, the logos are LA Dodgers, Yomiuri Giants (which I first thought was the Chicago Bears), Mystery Logo, and NY Mets. It's also possible that the logo could be some sort of team that has roots in New York since along with the Mets, the Giants and Dodgers both played there (even though the Yomiuri Giants just modeled their logo after the MLB Giants).
By far, the number one suggestion people have given has been a Porsche logo, since Jerry is a noted collector. But to me, things about it don't match with the Porsche logo.
In the mystery logo, the center seems much bigger and there seems to be more red compared to Porsche.
Other things that I'm pretty sure that the hat is not: NYPD, NYFD, Arsenal, Ferrari, Queens College where Jerry went to school, U of Maryland where Larry David went to school.