I am not a lawyer, but as I understand it, Steamboat Willie is entering the public domain, not Mickey Mouse. Mickey Mouse is a trademark, and trademarks don't expire.
I'm not entirely sure what that means here, but I've got a strong suspicion that the guy would still not be able to get away with a Mickey Mouse comic book.
Mickey mouse as a concept will enter public domain. Although with a trademark, it will be hard to do a Mickey mouse animation.
See Sherlock Holmes. Some, but not all of Sherlock Holmes is on public domain.
Also note that trademarks expire, they have to be renewed. In addition, you can lose a trademark. See Kleenex, escalator, zipper, bandaid, etc.
I'm not entirely sure what that means here, but I've got a strong suspicion that the guy would still not be able to get away with a Mickey Mouse comic book.