Mickey Mouse has had this particular problem for a long time. But it is not so much that it might be a flop. It is more that his image has to be squeeky clean, and they are often reticent to create a cartoon that chance making Mickey look mean or disrespectful in any way. Right or wrong, they felt like the storylines would be limited. That is why in later years Donald Duck (who was smart-aleck) and Goofy and others had more prominence, even in the shorts.
Mickey's true character, according to Walt, is an everyman character, and with a purpose of laughter. With that said, Mickey still had a valuable part, but one usually with whom the audience could identify. The surrounding characters were usually the comic relief or villains. And I think that formula still works, especially for shorts, which I think is really where Mickey Mouse belongs. Of course, there are few shorts these days. But there are some still, and he was in "Get a Horse," the one that ran before "Frozen." I think that was pretty successful, and had him in his playful 1930s style.
As for feature films, I think he has always been relegated to a cameo or short part whenever he appeared in one, unless you count "the Sorcerer's Apprentice" segment of "Fantasia," in which he was a lead player (but again in a short story) as a larger role. He made a cameo in "Who Framed Roger Rabbit" (with Buggs Bunny, who was contracted to be in the same number of frames as Mickey Mouse), as well as a few others. Of course, had character roles in "Mickey's Christmas Carol" and in "Mickey and the Beanstalk" (which was too short to be a full feature, so was combined with another short to be marketed as "Fun and Fancy Free").
I think that he is best in short subjects or cameos.
