Super Paper Mario was one of my favorite games.
MarioKart Wii is good.
It pushed another silly accessory (the wii wheel) but it was still ok.
If by "gimmick"you mean focusing more on tons of silly features and peripherals than games' stories, yes, Nintendo has let the Wii become a gimmick. Form follows function.

If their function is to promote a feature or accessory, it's pretty obvious. And that's a bad thing.
Nintendo's been doing that since forever. ROB. e-reader for the GBA. GBA-GCN games like 4 swords adventures. Donkey Kong Jungle Beat with the DK Bongos. Link's Crossbow Training. And the DS's touch screen and mic have been like a curse on DS software-- I would be completely happy if I never have to blow in the DS mic for any game ever again.
But yeah, Nintendo seems to be lacking a bit of substance. I would love to see a deep, beautifully written RPG-like game from them. But I'll be content with Tales of Symphonia's sequel from Namco.
But if you're someone who only plays games casually, Nintendo's model probably works... The thing is, by drawing in all these new people to video games (seriously, there are a lot of people picking up a game controller willingly or the first time), people are going to eventually want more than just casual games.
You may introduce kids to the fun of books with pop-up books or books with spinning cardboard wheels and paper shutters or even interesting textures... but eventually you want them to move beyond that and enjoy other things. Eventually the story or characters should take priority, not the way you can interact with the physical book. I kinda view the wii as more of a colorful children's book focused on physical interaction than an interesting, interactive novel I can read on my own and discuss with others later. <_< Guess which one I like better.
I want my games to have stories and developing characters.A mii playing tennis doesn't cut it.
I know they're trying to be innovative... And I can appreciate that they try. I'm happy that some people really enjoy what Nintendo has made.
Opening up WiiWare to lesser-known developers is a step in the right direction. It will allow more ideas to get out there. It will allow Nintendo to see what sells. I hope that can affect some of their future game development decisions.
It's a shame that WiiWare is so limiting on space... The deeper games usually take up more data than the glorified mini-games that seem so popular right now.

I really don't like having to learn a new controller with every game. That's complicated, not simple. It's not as easy on developers either.
They need more long-term vision.
And not everyone has lots of friends/family to play their silly mini games with.