Well, the movie is a comedy, hence the accents are supposed to be exaggerated. Look at the way Holly Hunter says SOB during the scene where Cage's character bungles the diaper robbery. She's originally from the south. I saw an interview with Cage where he thought his character was like a Woody Woodpecker type. Which explains why he's kinda wacky in the movie too. The Coens got kind of annoyed with Cage reportedly because they don't like to deviate from their script and he was won that liked to improvise a lot.