What is the postmodern lyricism in this poem?

"Lyricism" typically invokes something imaginative, beautiful, and/or poignant. In "Self-Portrait in a Convex Mirror," the lyricism lies in the multiplicity and fluidity of identity, a central tenet of postmodernism. In this poem, Ashbery embraces and celebrates the postmodern notion that the components of identity are highly flexible and mobile and thus easy to create and transform as we need. This variability and fluidity shines through in the poem's description of an artist positioning himself for a self-portrait:

As Parmigianino did it, the right handBigger than the head, thrust at the viewer And swerving easily away, as though to protect What it advertises. A few leaded panes, old beams, Fur, pleated muslin, a coral ring run together In a movement supporting the face, which swims Toward and away like the hand Except that it is in repose.

The undulating gestures of the artist and the pleasant jumble of textures symbolize the nature of identity itself. The somewhat warped sense of scale in the final portrait is not unsettling but real and true. Just as the individual identity of the painter becomes skewed and changes from moment to moment, so too does the painting. It is simply the nature of being and nothing to shy away from. "The soul establishes itself."
Later in the poem, the speaker observes (or rather, questions):

How many people came and stayed a certain time,Uttered light or dark speech that became part of you Like light behind windblown fog and sand, Filtered and influenced by it, until no part Remains that is surely you.

Once again, while uncertainty swirls around one's identity, it is not something to fear. The constant waves of time, people, facts, colors, and all else wash over us, but not in a tumultuous fashion. They simply "became part" of us until they no longer "are," a phenomenon as natural as "light behind windblown fog and sand." The poet views the ceaseless shifting and changing of individual identity as natural, necessary, and indeed beautiful, as indicated by the gentle imagery of the poem.