Been lurking for years but joined for this thread.
I agree that his art is primitive and childish BUT I can instantly recall a lot of his work. This tells me it's impactful and possibly iconic, at least in our world.
As far as his skating, Mark was one of first to skate rails and huge gaps, on he did it on huge boards with no noses. He was a consistent street innovator, he pushed hard for modern board shapes, and he also has serious vert skills. His contributions can't be discounted - we've ALL been influenced by him.
I completely agree that the outsized worship (Gonzitis) is very annoying. Truth is, his prime was probably 1985-1987. By 1988 the H-Street ams like Ray Simmonds and Ocean Howell were arguably more advanced.
He reputation as an irritating man-child is probably well-deserved, but at least he's himself and always has been like that.
His uncompromising individualism and innovation, however crude and sketchy it may be, is the essence of skateboarding. This is why I believe he is so celebrated to this day.