Taking it one step further
Clinton: I believe diplomacy should be the cornerstone of any foreign policy.That's so exact - when I saw the sketch, I didn't immediately get the joke. Or at least, I thought I got it, but I got it inverted. I thought it was Palin who had the zinger; then I realized, "Oh, no - they think that she's the one making a dumb remark, not that Clinton's vacuous hot air balloon was just punctured by sharp, down-to-earth reality."
Palin: And I can see Russia from my house.
In SNL land the second answer is meant to sound stupid. In the real world, the second answer is the better one.
When I thought about it more, though, I found one more level of inversion. Hillary's statement "I believe diplomacy should be the cornerstone of any foreign policy" sounds sage and magisterial, but it's totally false. It's partly due to the use of what Orwell called a "dead metaphor". Who really thinks anymore of what a "cornerstone" actually is? I don't wonder that SNL writers no longer hear the echo a Christian hears at that word: "The stone that the builders rejected has become the cornerstone", but it doesn't matter, because 'cornerstone' is a perfectly good word on its own, used in construction, and means "a stone forming a part of a corner or angle in a wall". The point is, it's an important stone set right down at the foundation, sometimes below the soil level, where it can't be seen, and the building rises on top of it. Obviously, it's got to be very strong to support the weight it holds up, and that's why "diplomacy" can NEVER be a 'cornerstone' of anything important.
Diplomacy is just talk, and talk is never strong enough to support a country or a country's relations with other countries. If it is, the structure on top of it will either collapse, or will have to be so light and flimsy that it isn't even worth bothering about. This is a forgotten truth in Canada, where we've indulged ourselves in the illusion that we can be important in the world with only "soft power" - a metaphor for bumptious moralism and pious self-regard, all expressed through nothing but talk, of course. We no longer keep much of an army or navy, and have persuaded ourselves that our ability to keep up appearances in the world is a result of the marvellous moral example we set, which all people acknowledge, admire and voluntarily defer to. In reality, like the Europeans, we rely entirely on the United States, and its willingness to maintain military and emergency forces, for anything remotely important. THAT is a genuine cornerstone, and we expend a lot of effort forcing ourselves to forget it.
So a cliché like the SNL one is just that - a windy, important-sounding piece of nonsense, that too many people take for reality.