That is one ugly box art. Besides the background, everything is basically official art slapped on in random places. Even the Blue Toad art is the same, with the red Koopa shell replacing the Baby Yoshi. Looks like some random photoshop from the internet.
Also, the fact that Nabbit is playable doesn't mean we should get excited about Nintendo trying out new characters. Nabbit was already programmed to be able to run through a level from the main game. The reason he can't collect power-ups and can't get damaged in the DLC is because Nintendo would have had to program that all into the character. So they didn't sit down and try to figure out a character to replace Mario. They saw Nabbit and threw him in there because it was easy to do.
Also, the fact that Nabbit is playable doesn't mean we should get excited about Nintendo trying out new characters. Nabbit was already programmed to be able to run through a level from the main game. The reason he can't collect power-ups and can't get damaged in the DLC is because Nintendo would have had to program that all into the character. So they didn't sit down and try to figure out a character to replace Mario. They saw Nabbit and threw him in there because it was easy to do.