The decision to go Full Metal Jacket and shave the dome clean isn’t strictly a matter of good and evil. In this case the shoulder angels and devils are more representations of impulsiveness vs. consideration, or positive vs. negative attitudes. It’s probably also true that on some level Mike thinks shaving his head will make him look tough, but that’s pretty unlikely.
Generally though, they represent exaggerations of Mike’s perception of his friends. He has made it clear that he mistrusts Amy, and Ryan is a virtual paragon of impulsive behavior Besides which, none of them would really argue that he should keep the wild hair.
This is a type of comic I’ve been wanting to do for quite a while. I think I wrote one last year, but it didn’t really work out. My favorite shoulder angel scene of all is the one from The Emperor’s New Groove, with Kronk debating killing the Emperor. Not the first one. The second one. The one he plays out in front of all of the other characters.
It’s something about the way the angel and devil end up agreeing, because self interest generally overrides the argument of right and wrong.