No, I consider it a gimmick if it's wholly unnecessary and doesn't particularly add to the gaming experience. The Wii remote set up, for example, seemed innovative at first, that was until you got your fill of Wii Sports. After that, it became refreshing when a game just used a more standard control scheme, although that led to the creation of too many peripherals and extra controllers to count.
When it comes to the 3DS, the 3D in absolutely no way adds to the gaming experience. It doesn't improve how the game looks, and if anything, it holds the 3DS back since they opted for a special low res screen to support this 3D option rather than focusing that development time and money into improving the console's specs. The 3D is as gimmicky as gimmicks go.
Hell, with the Wii U, the gaming tablet had a chance to be innovative, but just became that thing that drove up the price of the console for a very small handful of games that utilize it well. In the long run, it's unnecessary, more often than not fails to improve the gaming experience, and feels gimmicky like Nintendo felt like a more classically designed console wouldn't sell. Or rather, they seem to think having some quirk that makes their hardware stand out will make the difference when clearly, that is not the case.
And their philosophy may be "what's fun", but I think fun and them are having issues keeping in contact these days as Nintendo struggles to turn out quality content that is, you know, fun, in a timely manner this generation, both on the 3DS and the Wii U.