What is the narrator's attitude towards the subject matter of "The Miller's Tale" in Chaucer's The Canterbury Tales?

Although I am a bit confused as to what you mean by "the narrator," I am going to assume you mean "the host" of The Canterbury Tales (who travels along beside the pilgrims and urges them to tell their tales).  Usually, the Host is more than glad to hear a new tale spun for them; however, this is not so in the case of the Miller.  In fact, the reader finds out within the "Words between the Host and the Miller" that our dear Host tries to stop the Miller:

Our Host perceived at once that he was drunk / And said, "Now hold on, Robin, dear old brother; / We'll get some better man to tell another; / You wait a bit.  Let's have some common sense." (Chaucer 103)

Here we learn the Host's main attitude toward the Miller:  one of annoyance.  We also learn of the Miller's reputation as a drunkard and teller of bawdy and lewd stories.  The irony here is that we find out the Host's attitude before the story is told.  The Host, it seems, knows the Miller well.

What can I add?  The Miller had begun. / He would not hold his peace for anyone, / But told his churl's tale his own way, I fear. / And I regret I must repeat it here, / And so I beg of all who are refined / For God's love not to think me ill-inclined / Or evil in my purpose. I rehearse / Their tales as told for better or for worse.  (104)

Here we add another attitude of the Host to the Miller's subject matter:  the attitude of regret.  My guess is that our Host much prefers the exemplum of the Pardoner than the drunken spout of the Miller.  He is generally a gentle Host and approves grandly when his followers learn something from these stories he has requested.  The reader sees, then, why the attitude of regret can be found here.  For thus follows the most raucous fabliaux of English literature, and one that my students all can't help but love.