People lie in hammocks when they want to relax, when they want to get away from daily life and work, or when they simply want to go to sleep. This is the significance of "lying in a hammock." This is an activity in which the person is inactive and not doing any work.
The speaker observes the sleeping butterfly, an animal doing essentially the same thing he is doing. He also observes horse droppings baking in the sun. The speaker could be using these images in comparison with himself, noting how lazy he is. And the last line, "I have wasted my life" would seem to support this interpretation. Lying in a hammock, he is about as useful as a sleeping butterfly or a pile of horse droppings. And at least the chicken hawk is flying, doing something active, looking for home.
However, the speaker could be making a sarcastic point. By "wasting" his time, he is able to relax and observe interesting things in nature (sans the horse droppings). In fact, only by taking time to reflect is he even able to discern whether or not it is good or bad to take time to relax in a hammock. He might be implying that to get into a state of serene contemplation, one must be in a relaxed, meditative state not unlike lying in a hammock.
The poem is straightforward if we read it literally. To be lazy is to waste one's life. But, if there is even a hint of sarcasm in the final line in particular, then the meaning changes. But in any case, the fact that the speaker is relaxing in a hammock is key to either interpretation. Relaxation breeds laziness. Or does relaxation breed contemplation and appreciation of life?