The skylark's song is better than all the human music ("measures") in the world.
Sure, we can plan and scheme and make "delightful sound" but we can't stack up to the skylark.
Better than all treasures That in books are found,
Guess what? Our books don't cut it either. All the poems and ideas and novels we've stored up are "treasures." But still: they don't match the skylark's song.
Thy skill to poet were, thou scorner of the ground!
Finally, he comes out and admits it. The poem that the speaker's writing will never be as good as the song he's writing it about. A little bird can make music beyond his description, beyond his power.
The simple song of the skylark is more wonderful than even the best human poem.
This is the bittersweet irony under this poem. Even if it's moving and beautiful (and we think it is), it's all about the ways that poems fall short, the ways that they fail to measure up to the beauty of the world.