I'm not really convinced 'Nintendo games' as a whole are designed to be easy, they just have some IP that tend to be more relaxing and forgiving to play, and so are well suited to beginners, and others with a decent difficulty curve throughout. It depends on the genre too- I play a lot of strategy games but found the final missions on the Advance Wars games to require a reasonable understanding of everything in play, and various earlier Fire Emblem games require you to have a reasonably strong force and thus have made decent decisions over the course of the campaign too. I wouldn't put them in the same bracket as Yoshi platformers, any more than I'd put something like Xcom in the same bracket as something like Skyrim that's designed to be far more accessible.
Even with Mario games, the later stages of the 2D ones don't really pull their punches, and Donkey Kong Returns/TF doesn't either. I'd say there's a variety in terms of expected skill level by the end across even just their platformers, let alone the various IP and genres.
Zelda is one where you get out of it what you put in- rush for the end and the game will be fairly challenging, explore and the experience gained will help as much as the incremental upgrades.
