Its a hot take, but not wrong. If a mumble rapper is defined by a performer who literally garbles and mumbles lyrics to a great riff or beat? Then not only was Kurt a mumble Rocker, Bone Thugs were mumble rappers. Literally the joke made about them, was rapping so fast no one understand what they said. Incoherent lyrics = mumble rap...nuff said.
With that typed, I don't care if you're lyrical spiritual, miracle either, If your beat is trash, I don't care what you have to say. Nas is the only exception, and even then, massive eye rolls are induced when only 3 songs deep into most of his albums.