I'd say NSMB series is more a conglomeration of art elements from SMB, SMB3 and SMW. It takes the World environment look well ditching the more unique looking things like the flip blocks. 3 has the mentioned stage look, tons of wood, that's ditched and outside of that it shares a lot with SMB, and often where the two differ NSMB takes from SMB, but it does give us all but two of the world themes used by NSMB. It takes a noticeable amount from SMB, yeah not the rock ground, but for example the brick blocks look far more like they do in SMB then in SMB3 it also has the indestructible stone-ish blocks that make up pillars and and stairs like the end level ones in SMB. These were replaced with wooden ones in SMB3 and are almost nonexistent in SMW, but their back and sporting the same design as in SMB. Speaking of level ends the reuse of SMB's is notable in that both SMB3 and SMW did all new ones. That and level, notice who every single NSMB's 1-2 is an underground level? Yeah riffing on SMB, because SMB3 sure didn't have an underground level as it's second and neither did SMW.
The result is to make, depending on your point of view, Iconic visuals of Mario stripped of the distractions and focusing on what unites the games visuals, or a generic version of Mario visuals with none of the unique charms of the individual games it steals from.
And this iconic/generic is very intentional. NSMB for the DS was basically sold as “Hey remember 2d Mario? Here' he is again largely unchanged but with new levels!” NSMB has largely stuck to this motto. Not that they don't bring new things, but they don't want it be too front and center, they want it to feel familiar, they want it feel like more of the same.
Where SMB3 and SMW well keeping a lot of similarities to each other and SMB also wanted the feel to the them to be different enough that they felt like distinct new experiences and games hence SMW redesigning the fire flower and both of them changing how the end of the levels look, something most direct sequels would keep the same from one entry to another.
NSMB works from the assumption, that well you do need new things to spice up each entry at heart what most people want is something that is overall the same but with new levels, or even remixed/refined versions of old levels. Hence the focus on remixes for the sound track, hence why every level 2 is an underground level and every second world is a desert. Hence why we don't see the massive number of redesigns that World had or the stage play look of 3 and instead a far more standardized art style from game to game, and even standardized level ends and physics. It's not that the makers can't come up with new things to fill those slots, they actively decided not to, because they think doing otherwise would put people off.