Well yeah, it's only a matter of time before the Earth gets hit by something, but something the size of Mars seems pretty unlikely. We haven't been hit by something that big in at least 3-4 billion years (when the universe was a much much much much more hectic and chaotic place), and Earth only has another 5 billion years or so left.

It's not just a tiny tug that will send a large, planet-sized object from the Oort Cloud into the inner solar system. It would take a huge push, according to Isaac Newton. Most of the things out there are smaller and more reactive to small tugs, which is why we see so many comets and so few giant ice planets coming out of nowhere.

In any case, I don't see interplanetary collisions as a serious threat to our species. In all likelihood, a huge collision is not going to happen in the next couple hundred years, and if we don't wipe ourselves out by then I'm sure we'll have the technology to prevent or at least flee such a collision.