Meh. I guess the Mushroom emblem just doesn't fit him (like it fits Bowser, either...).
Edit: You know... every character could have their own unique emblem and victory tune. This obviously won't happen, as Bowser and Mario already share the Mushroom, but it isn't impossible. Mario and Luigi's emblems could be changed to M and L, like Wario's W. Peach could keep the Mushroom, as she rules the kingdom (unless Toad came by and stole it). Bowser's would be that silhouetted face of his, as in SM64 (die.). Yoshi could keep his egg. Mario's victory would stay the same, Luigi's could be from SMB2, Peach has something from SPP that could be used, Bowser could have the classic SMB game over tune, Yoshi's should be changed to SMW level clear music (just because). If Dr. Mario were to stay his emblem would be a Megavitamin. For the Zelda characters, each would have their part of the Triforce filled in with the others empty. Meta-Knight's emblem could be his mask. It'd work, all of it would... there's always something.