Clouds are supposed to be gentle. The speaker opens by calling that idea a lie: we're told clouds hold our dreams, taste like cotton candy and feel soft as cotton, and every child imagines jumping onto them to rest. The poem then turns sharply at "But—." When the speaker actually tries it, there's nothing to hold them, and they fall straight through.
After that, the clouds change. The soft white goes grey, and the fall becomes a storm, with thunder in the ears, flashes of light blurring the sight, and chaos that sets fire to whatever lies in its path. The short, plain lines at the start give way to a more violent, overwhelming picture.
The speaker seems to feel betrayed and frightened. It reads like losing a childhood comfort, or finding out that something promised as safe, perhaps a dream or hope, can't carry real weight. The mood moves from innocent and sweet to disappointed, then to panicked and destructive. The reversed, upside-down title may also hint at a world turned over.