The system itself is considered a family console due to it's innovative control-scheme. Nintendo has been taking advantage of this by releasing many family-oriented party games or games that would appeal to all audiences in an effort to garner more attention from people who previously didn't play games very often (or at all)
However, the system, in reality, is just like any other in terms of "maturity". The real question should "Do we think more mature-rated games have been released for the system lately" since, in effect, it all boils down to what kind of game the developers decide on producing for the system.
Anyway, yes, I do beleive a lot more games focusing on teens and/or matrure audiences are being released lately. That's to be expected, though. The Wii is just passing it's "first year released" mark and developers are finally getting used to the Wii's capabilities and now know it's popularity.
Within this next year and beyond will be the time when larger, more involving games will begin to be released for the system. And yes, that no doubt will include some "mature" titles, as the Wii has already had a few in the past. The Wii will cater not only to the family, but to the individual gamer, as well.
It'll have to, eventually, if it wants to maintain it's current popularity these next coming years.