Only George speaks with a New York accent. Elaine is exempt, being from Baltimore (Townsend). Jerry and Kramer are ostensibly native New Yorkers, though you’d never now it from their speech. But, their characters don’t seem to suffer.
But, George, and his parents, are definitely New Yawkahs! To my mind, MANY of George’s best lines are funnier because of the accent, especially the dropped “r’s”. Some examples:
From nearly every episode:
Hi, I'm Goorge.
George to Keith Hernandez in The Boyfriend:
Keith! I'm Jerry's friend. The guy from tha lockah room. I'm the chuckah!
George to Anna in The Little Kicks:
I’m a bootleggah!
From The Fire:
Jerry: Did you knock her over too, or just the kids?
George: No, huh (her) too. And huh mothah.
George to the doctor in The Pilot
Cancah? Is it cancah? Do I have cancah?
From The Susie when George has been trying to win back Allison (Kramer).
Kramer: George, I thought you were gonna change.
George: Fa huh. Not fa you.
Does it make a difference owah would Goorge’s charactah work just as well without the NY accent? Owah, is this too silly to even considah?