Even if you disagree with this argument, there's a significant difference between this era and generations previous. In true Revenge of the Nerds style, the music develops without the real-life, socially-derived elements that defined previous generations' "lowest common denominator" styles.
Tumblr rap can describe a diverse range of artists, but the subgenre is primarily defined by its initial audience on Tumblr, the engine that propelled the act into magazine headlines, even if those artists transcend that audience later on.
Tumblr-popularized rap artists could arguably include Flatbush Zombies, Raider Klan, Kitty Pryde, etc. There's nothing inherently wrong with any of these artists, but what differentiates this scene from past scenes is that it's experienced largely through social media on the Internet. And what that means for the bulk of the artists is that it's as much about the Internet-identity of the listener as it is about the artist's identity. It's music specifically designed for reblogs.
What this means is that it's also music for a generation that is especially self-aware about its own image, thus the focus is on fashion, on "visuals" (music videos), on signifying awareness of certain eras or scenes that are perceived to have some cultural cachet. What isn't of concern: dancing, lyrics, a sense of pop immediacy.
Music that had been inherently social pre-Internet—the lyrics kids shouted in the hallways at school to annoy teachers, the dances you had to learn and perform, the way you responded to the groove—in pure Tumblr music, that is all irrelevant. Online, no one can see you dance.