I defiantly think they can be called soulless. I don't think the core gameplay has ever changed much in 2D Mario games; but the main thing that made each of them feel different were the levels. In NSMB they have been using the same worlds in every game and they barely add anything new to the world. Not only that but the art style is the same every game with reused models, animations every thing.
Varying stage themes have never been particularly prevalent in 2D Mario titles, SMW has less area variety than SMB3 as well as using old settings with a new skin, even the visually praised Yoshi's Island has the same basic world setups of plains, jungle/forest, snow, clouds and caps off each world with a castle.
I'm not going to say that there's anything good about NSMB sticking to those same old tropes and not bothering to even overhaul them a bit until NSMBU but from my standpoint the SML duo are the only ones that truly went out there with unique settings.
2D Mario has always been pretty safe, they tend to throw the more varied or unique locations to their other platforming franchises like DK or Wario.
Speaking of Dinosaur Land for a second its main appeal thematically should really be viewed as the new enemy types involved in them since they're pretty unique to SMW and the sprawling map angle, the actual stages and locations are token Mario settings with food names, vanilla dome isn't an ice cream cavern, it's just a cave.
After striking success with NSMB they've been hesitant to change any of the art direction substantially, as silly as it sounds it's as if they perhaps feel the more casual audience might get scared off by a visual tour de force spewing artistry out all over the place and looking completely different to what they're used to.
While some would prefer 2D platformers to be sprite focused and rich in art style I imagine there's also a large audience out there that think if it's not with 3D models it's not as good because more traditional 2D is "antiquated" or some such angle.
Considering the success of NSMB to their many other platformers, some of which with distinct visual styles like Wario Land: Shake it, Kirby's Epic Yarn and even the rpg/platformer hybrid that is Super Paper Mario then maybe there's a method to this madness and that NSMB is intentionally designed to be basic and inoffensive visually (which in turn becomes offensive to some for lacking ambition), or i'm just talking nonsense taking shots in the dark.
But I do actually expect NSMBU to be the last 2D Mario for a while and possibly the last with the NSMB name tag, four SMB games and four NSMB games would make some nice synchronisation except these last ones are deliberate throwbacks that weren't made to redefine the genre, that torch for the Mario series seems to have been passed to the 3D titles. Four "new" games since 2006 isn't that bad, with the way people go on about them around here you'd think they made double that.
Hmm, maybe one day we'll get New Super Mario All Stars.
Also Chocolate island is a brown version of the standard stages with lava turned chocolate brown in the caves, if that's unique then I guess Desert Dessert is as well.
I'd say unique is more like the instrument themed desert in Rayman, it actually uses the musical theme in the stages as well.