Mickey Mouse is trademarked as a word and as a picture and all kinds of derivatives like the silhouette of his head or the design of the eared hats. You can't use him as a symbol of your business or to suggest that your product comes from Disney, but none of that will stop you from making a new creative work featuring Mickey as he appeared in Steamboat Willie any more than McDonald's can stop you from writing a book where characters eat a Happy Meal.