Really the meaning of this song is simple. He never wants to grow up. When the lyrics state, "Everything in the back of my brain, Told me that I would be sick, When I'm out there," states that the world is a crappy place, and he doesn't want to be an adult and face the world. It's titled Mickey Mouse because that's one of the biggest child icons, which he is dear to and, again, doesn't want to grow up and go out in the world.
Really the meaning of this song is simple. He never wants to grow up. When the lyrics state, "Everything in the back of my brain, Told me that I would be sick, When I'm out there," states that the world is a crappy place, and he doesn't want to be an adult and face the world. It's titled Mickey Mouse because that's one of the biggest child icons, which he is dear to and, again, doesn't want to grow up and go out in the world.