Out of interest, I googled the list of winners
Of the ones I know (which isn't most of them I'm ashamed to say; many don't write in English, I think) I'd agree that Tolkien's prose doesn't compare well. To say that Tolkien wrote as well as Steinbeck, Hemingway, Sartre, Pinter, Heaney, Yeats or Shaw is just wrong. Having said that, there are a few others there that I don't think deserve it as well, such as Morrison and Kipling.
Tolkien wrote great stories, not great literature. As I understand it his intention was to create an English mythology because we don't have one, really, except Arthur and that's very parochial. He succeeded in that for sure, but 'great' writer? No. Rowling comes closer. She has far greater subtlety and depth of character.