I dunno, I guess I don't feel strongly one way or the other about the graphics in NSMB as some here do. Would better graphics be nice? Sure. But it would do absolutely nothing for the game itself. I might have a few moments of "ooh, that looks pretty sweet," but after that I would forget about it and focus on the game itself. And that's where the Mario series has always excels, particularly the 2D series - pure gameplay that shines above all else, which is the way games should be. Look at Minecraft, it is litterally a bunch of blocks, and yet it is probably one of the top 5 most fun games I've ever played, and a smash success.
You could say why not have both? But also keep in mind many developers that focus too much on graphics and horsepower often seem to compromise on the quality and scope of the gameplay. That is why you often see these modern games that look pretty, but are very linear, structured, and short.
Again, for me, the issue with 2D Mario is recycling the same worlds. If there were some interesting new environments, I doubt too many people would be whining about the graphics in the game. In fact, I think that is where the roots of the complaints come from.
That's not to say the game should look like garbage either. It should look passable based on the capabilities of the hardware. And NSMB certainly fits that bill.