The biggest problem with the perception of the art in NSMB branded games, is that a lot of people seem to have this notion about graphics and art direction:
1. If I like this art, it is a masterpiece of craftsmanship and demonstrates the creators are genius.
2. If I don't like this art, it is lazy and was made in ten minutes, showing the creators hold me, the player, in contempt.
The art direction in the NSMB games is extremely specific, and hardly generic. Some people don't like it as, it seems, they are only personally excited by impressionistic and stylized art. That's fair, one man's ceiling is another man's floor. But the art style in NSMB is nothing more than the friendly, inviting style of rendering Nintendo has been experimenting with for over a decade in the primary concept art for their mainline all-ages games. It started with the master character renders for Super Mario 64.
It was then greatly refined during the Gamecube era, when Nintendo created a whole slew of 'reference models' for virtually all of their characters in every game. Smash Bros. Melee was the first game to heavily demonstrate this, and its art direction is not unlike the smooth, polished rendered style seen in NSMB. Another series to follow this aesthetic is Mario Kart - which starting with the N64 version, has had practically the same art and rendering style in every single game.
The mistaken notions surrounding NSMB are pretty flippant at this point - witness how many people assert that the art assets in each game are literally the same. Which is false. NSMB U does not 'reuse' assets from NSMB Wii - every piece of asset, every model, is new and not merely scaled up to HD. NSMB 2 on the 3DS does not reuse any actual assets. Each game uses new assets, but the art style and rendering is consistent.
As for the primary question with variety of worlds... I have had the impression that what Nintendo is doing with the NSMB series is a matter of iterative refinement. First there was NSMB DS. It was a prototype, and it is widely agreed to be very crude in comparison to all the other games. NSMB Wii took the same themes as the DS game, recreated all the assets to a higher standard with a lot more detail and quality.
NSMB U follows NSMB Wii directly. Therefore, it does not reuse the same world themes and art elements, but adds new themes. NSMB 2, meanwhile, seems more like bringing the refined Wii version to a portable format, thus it uses many of the same themes, just with much higher level of quality compared to the original DS game.
I personally don't agree with all of their strategy. I would have considered the DS game done, and gone ahead with new themes for NSMB 2 - but I think I get what Nintendo is doing, and it makes a kind of sense. But then, I am not offended by the art style and it is quite appealing for me. That doesn't mean I wouldn't like to see a mainline Mario game with other visual styles. Yoshi's Island is my favorite platformer of all time, and part of its appeal is its particular art style. You can like more than one thing, you know.