Well, in this interview in Iwata Asks, that did appear to be the case.
http://iwataasks.nintendo.com/interviews...ators/11/0
Nomura - "Well, just as I was working on FFVII8, Mario 649 was released. The fully three-dimensional spaces and the freedom you had to run around them had a big impact on me. When I told my colleagues I wanted to make a game like that, they said 'but Mario's already a world-famous character. It would be impossible to start from scratch with an all-new character.'"
Iwata - "They didn't think you could compete with Mario?"
Nomura - "Exactly. Somebody even said 'The only way you could do it is with characters that are as well known as Disney's'. That really stuck in my head, so when I heard we could be working with Disney characters, I naturally jumped at the chance."