I feel like back when New Super Mario Bros. DS released, very few people were upset about the art style, music, or similar structure to Super Mario Bros. 3... because it was the first traditional sidescrolling Mario in over a decade. Come NSMBU, four NSMB games in 6 years, and a lot of people feel burnt out on those similar concepts being reused. Shared assets are typical, and even shared music tracks (New Super Mario Bros. Wii is the last NSMB game to be mostly new music tracks). But if in 2012, this was the first ever traditional 2D Mario released since Super Mario World (or Super Mario Land 2 depending on your definition of "traditional"), would it be generally considered a high level Mario game and one of the best in the 2D series?
Consider that when NSMB came out, a big emphasis on it was a "back to basics" approach by having Mario only have the Fire Flower power-up return, and no trademark flying power-up, no Yoshi, instead opting for a rare Koopa Shell power-up. But since then the series brought back the Koopalings, Yoshi, and added several flying power-ups as well as power-ups of other kinds. It felt like the series had to spend several games building back to where the series left off in the early 90s. While NSMBU is seen by some as having the best level design in the sidescrolling series, the NSMB fatigue is real for others and although it's usually seen as the best NSMB game in general, they're often all grouped together into the "disposable" category.
Consider that when NSMB came out, a big emphasis on it was a "back to basics" approach by having Mario only have the Fire Flower power-up return, and no trademark flying power-up, no Yoshi, instead opting for a rare Koopa Shell power-up. But since then the series brought back the Koopalings, Yoshi, and added several flying power-ups as well as power-ups of other kinds. It felt like the series had to spend several games building back to where the series left off in the early 90s. While NSMBU is seen by some as having the best level design in the sidescrolling series, the NSMB fatigue is real for others and although it's usually seen as the best NSMB game in general, they're often all grouped together into the "disposable" category.