Nah Rocket never had an accent until Marvel Vs Capcom 3 gave him that cockney one, and since so many people have only heard that voice, that's what people think that's the voice he should always have.
I would assume the accent would be in part due to the fact that his character is a tribute to the Beetles. But since he was from the Dakota hills in the song, maybe having a hillbilly twang would be more fitting.