Yes but irony is about the outcome you're addressing being opposed to the common or expected outcome. So what would the opposite of drawing a tree on paper be? We don't usually draw pictures of paper on trees, or trees drawing on...us? Or something like that? There's no opposition to the expected (or even literal) there. Cardboard is made out of Trees, so a tree made out of cardboard is at least conceptually ironic (although not unexpected, it is opposite to the "norm"). I'm not trying to be a Know-it-All dick or anything. It's just such a commonly misunderstood concept....or maybe I'm missing a message here.