A) Johnny Depp sucks.
B) Johnny Depp isn't known for his voice or for being a voice actor.
C) I watched this entire movie without even realizing JD was even involved until I saw his name in the end credits. Upon becoming aware of that information it actually made me like the movie less.
The fact that you could watch the whole thing without realising it was Depp, and enjoyed it less only given the revelation that it was him, sorta belies the first two points.
If he was a crummy voice actor, wouldn't you be able to hear him just phoning it in? If he just sucked generally, wouldn't you have disliked the film and only realised *why* later, when you found out it was him, instead of having the information itself be the disappointment?