Three short lines, and the whole poem is a mild grumble: the speaker doesn't get along with camping, and the reason is their sinuses. That's it. There's no grand statement about the wilderness, no sunset, no campfire. The humor comes from how small and practical the complaint is.
Think about what we usually expect from a poem about the outdoors. Camping tends to show up as peaceful, romantic, a way of getting back to something real. Here the speaker quietly turns that down. Pollen, dust, damp air, smoke: the poem doesn't say which is to blame, but we can imagine the stuffy nose and the sneezing. The phrase "doesn't agree with" is polite and a bit old-fashioned, like someone declining a second helping of food, and that understatement makes it funnier.
The mood is dry, wry, and lightly self-mocking. The speaker sounds honest rather than bitter. The poem seems to say that the body gets a vote, whatever the postcard version of nature promises.