Actually “fluency” is something of a misnomer I committed. What does good style mean in a conlang like Klingon? People clearly do differentiate between good Klingon and bad Klingon; on what basis do they do so, when the language is made up, and we don’t have any utterances from its creator longer than a couple of lines of barked orders?
It would be a challenge to get a linguistics department to take it seriously. It would be even more of a challenge to get a literature department to take it seriously, and it would be the kind of thesis that could do with input from someone dealing with rhetoric (which linguists tend to think beneath them). But there’s a PhD in it, for sure. And it spans across mental models of style, and fads in English prose style, and translation theory; in fact, it reaches into the theory of aesthetics.
It’s the question that got me into linguistics, btw (in its Lojban iteration). And I sort of have an answer for it, as the answer linked shows. But it can be filled out a lot more than that.