What's the literary world's view of JK Rowling?

Depending on what you mean by orthogonal I disagree. Popularity is by no means a guarantee of quality, nor does it indicate inferiority.

Given that “orthogonal” means “statistically independent,” I’m pretty sure you don’t disagree :).

Orthogonal: At right angles to. Independent of. Irrelevant to.

A wall is orthogonal to the floor, unless your walls slope. What shirt you wear is orthogonal to the wind speed on Venus.

So, yes, popularity is orthogonal to quality.