When a piece of literature is about literature in general, things can get a little tricky. What it means for a poem to be about poetry is that its content somehow reflects on the process or craft of its creation. Take, for example, a song about writing a song: "Ohhhh, it’s so hard to write a soooong, about how much I loooooove yooouu, oooh ooh," etc. It’s a song about writing a song. In the same way, any poem could be about the process of writing a poem or artistic creation in general. In one way, "The Tyger" is a poem about writing a certain kind of poetry. Blake is very interested in visionary poetry, the kind that communicates deep truths about the universe, often concerning the divine or a higher power. Knowing anything about these subjects is really hard, so "The Tyger" can be read as a poem about how hard it is. If we think of the Tyger as the divine or as the knowledge of the divine, and the creator ("he") as a poet, then the poem is about the speaker questioning how a poet could ever "frame" or possess the knowledge of the divine, let alone write about it in a poem.
Blake’s poem is about the difficulties of saying anything at all about the divine, let alone conveying some sort of universal truth about it; in doing so, Blake makes a postmodern statement about the limitations of art in general.
The Tyger represents the perfect visionary poem – the quintessential linguistic rendition of the divine; however, the "he," or the visionary poet, is never able to completely capture it.