That's just an assumption though. Maybe Nintendo didn't have any ideas for 2D Mario, that's why it's been stale. NSMB2 at least got the gold coin priority which changed the game mechanically. NSMBU was an original game too, contrary to popular belief. It introduced better level design and the acorn suit did provide a new way to play the game. Albeit not a huge one, but innovating a 2D Mario is not nearly as easy as it is with 3D Mario, because there are many more limitations.
It's not an assumption. It's pretty obvious that a lot more could be done with 2D Mario, and the 3D Mario games prove that Nintendo has no shortage of ideas to put into Mario games. World themes like circus and temple (which are in 3D World) would have been easy to implement in 2D Mario.
Music is another thing that highlights how badly Nintendo treats 2D Mario games. NSMB2 relied almost exclusively on remixed music from the previous NSMB games, and said remixes usually didn't do much more than adding more 'bah bah' sounds to the compositions. While NSMBU does have a new theme, it uses remixes of that one theme throughout all of its worlds.
Enemies are another blatant example of Nintendo's poor treatment of 2D Mario games. NSMB2's only "new" enemy is a drybones variation of the piranha plant. NSMBU offers a little bit more than that, but that still pales in comparison to the new enemies in 3D Mario games, especially when it comes to bosses.