That may be true in some cases, but not in all. In particular, the Onion school shooting joke strikes me as a satire of a strawman. Whatever humor can be derived from ridiculing a stupid version of a position (like "school shootings are unavoidable") is, at least for me, made flaccid by the counterproductive nature of the satire. In other words, when the satire ridicules a dumb argument that not many serious people make, it's not so funny.