There's at least some correlation. How strong that correlation may be is debatable, and there are always exceptions and extenuating circumstances. But writing is an expression of thought, and as such, its elegance -- or lack thereof -- often reveals quite a bit about the mind of its author.
Correlation's don't have to be linear. The correlation could easily be negative at the high end of the IQ scale and positive at the low end of the IQ scale. Great writers don't expose their minds they shape the readers mind though both elegance and fnord.
"The correlation could easily be negative at the high end of the IQ scale and positive at the low end of the IQ scale."
This is conjecture, though. And, while entirely possible, it doesn't necessarily seem to be borne out by available evidence.
"Great writers don't expose their minds they shape the readers mind..."
This is a false dichotomy. "Exposing" one's mind, and influencing someone else's, are not mutually exclusive activities. Furthermore, shaping a reader's mind is not the sole province of the "great" writer. Terrible writers are often capable of influencing people. While I would agree that stronger writers are probably more capable of influence, influence itself is an after-effect of writing and an observational artifact. It is not intrinsic to the written product.
My point is that, since writing is necessarily an act of expression, the strength of one's expression reveals something about the strength of what one is trying to express. It's hard to cleanly separate the two.
Now, I'll readily concede that intelligence is probably not the most important variable in the quality of one's writing. Things like training, education, practice, exposure to other writing (via reading), etc., are far more important. But intelligence is in the equation. Exactly how significant it is is up for debate.
the strength of one's expression reveals something about the strength of what one is trying to express
My point is simply that someone like mark twain is able to bend otherwise poor grammar to fit a number of ends. Because the English language so redundant and people are used to imperfect prose you can convey some types of subtext though gibberish more cleanly than you can with perfect prose.
There are plenty of hacks that confuse the issue, but don't confuse pretense with actual mastery.
"My point is simply that someone like mark twain is able to bend otherwise poor grammar to fit a number of ends."
Mark Twain was a master of the English language. We shouldn't confuse his characters' "poor grammar" with his. Huck Finn talks like a redneck because Huck Finn is a redneck. That's not Mark Twain's voice; that's his character's voice.
By contrast, read an essay by Mark Twain. The language and style are markedly different from those of his novels. Sure, there are some allowances for colloquialisms. But those are stylistic choices, not failings to grasp the mechanics of grammar or syntax.
"you can convey some types of subtext though gibberish more cleanly than you can with perfect prose."
It's not about perfect prose; it's about clarity and economy of communication. There's no such thing as "perfect" prose, anyhow. But there are such things as clumsy prose and clear prose, or correct syntax and incorrect syntax. Generally speaking, it's preferable to be on the latter end of those spectra unless writing for particular effect.
