It may be art, but being art doesn't make it deep or emotionally impactful. Even the people who like mumble rap often say they just care about the beat, not the vocals.
That, to me, means it could probably be generated by an AI. And I'd say the same of any genre for which repetition matters more than lyricism or storytelling. I could see a machine coming up with Lil Pump's Gucci Gang[0] much more easily than Wu Tang's Triumph[1].
Mumble rap is a totally legitimate art form. I just tend to not be a huge fan of most of the song structures and compositions, personally.