I don't think NSMB Wii is a casual title, nor is it hardcore. It is just a game (a franchise probably) which appeals to hardcore and casual gamers at the same time. And my guess is that the game was designed to do just that.
It appears to me that calling games "casual" is becoming more and more of an deprecative term. So I feel that by calling the Wii and it's games casual (wether they really are casual or not) is just a way of Analysts and perhaps the so called "hardcore gamers" to express their disapproval for Nintedo's strategy (and success).
I think this is the case because a lot of people find this success sort of illegitimate because it is not based on what has been viewed as the business standard (superior technology, graphics... higher complexity etc.) in the past.
