The speaker sounds like a butterfly, though the poem never says so outright. The clues are the "silk-parachute plant" (a milkweed, judging by the "bitter milk"), the chrysalis, and the final mention of Mexico, where monarchs are known to migrate. Having tried to fly, the speaker decides it was all a mistake. Its path is erratic, unlike a dragonfly's or a hummingbird's, and it feels too weak to climb above the hills. So it wants to quit and crawl back into the chrysalis.
The tone is self-mocking and a little rueful. The speaker lists its flaws with a shrug, as if packing up a failed project. But the closing question, "Whatever made me dream up / some place called Mexico?", gives the game away. The dream is still there, and the speaker is embarrassed by how big it is. The mood is a mix of doubt, humor, and stubborn longing, something anyone who has set an impossible goal might recognize.