Kramer was in Mad About You, and they reference "Jerry across the hall", but then there's an episode of Seinfeld where George gets stuck watching an episode of Mad About You.
https://www.youtube.com/watch?v=H-Bh8eo60Ns
Also, Mad About You was not funny to me when it came out (I was 12-18 while it was on). Not sure what tying it into Friends/Seinfeld world did for any of them.