that Spanish *Ay* can be hard for a non-native to pull off with any conviction
Maybe, but Angel spent several minutes of the reading repeating what he wrote in the book’s afterword, about how the music of Poema del cante jondo had felt native to him.

Those difficult poems leave me feeling sort of dumb when I don’t get them.

All difficult poems, or only certain kinds? Or is this feeling of intellectual inferiority something you use to decide whether or not a poem is difficult?

I ask because I do see a sharp distinction between poems such as the Ashbery one with which I started and the average poem by someone like Ralph Angel (or Reginald Shepherd, for that matter). I think Ashbery and his epigones would be pleased to know that us less-sophisticated types feel excluded by his “extensive linguistic play” and “parodic tone,” which seem tailor-made for an in-group of fellow sophisticates. Unlike you, I am never intimidated by stuff like this — maybe I should be. As you can probably tell from my lame attempt at an Ashbery parody, I do feel his poetry is “difficult… in an attempt to cover up [its] vacuity.”

As for Kooser, I very much agree with Dana Gioia’s point in the linked essay (and it’s not often I agree with Mr. Gioia about something): an apparently easy poem can be very difficult to write. A superficially difficult poem, by contrast, can be relatively easy to write, especially if it remains in the realm of pure intellectuality and doesn’t attempt to make any deeper appeal to the spirit or emotions.

That’s one way in which the easy and the difficult can complement each other. Also, what one person finds easy to understand another may find impenetrable. Snarky detractors of Ted Kooser or Mary Oliver may, in fact, be out of their depth and unwilling to recognize the fact. The quality of attention needed to really hear certain poems may simply not be available to those who spend all but ten minutes of every day inside, breathing each other’s stale air.