I'm cynical enough to suspect that any nation that decided it was in its national interest to meddle in another country's election would probably do so. It would not surprise me at all to learn, for example, that the U.S. had meddled in the elections of several other countries, including some that thought of themselves as our allies at the time. In fact, it would surprise me very much if I were to learn that we hadn't. So yes, I suspect that this sort of thing happens frequently and that many countries do it.
It makes perfect sense that Russia would aim to de-stabilize the U.S. by spreading dissent and attempting to influence our elections through fake news. Even a cursory study of our weaknesses would reveal pre-existing divisions, hyper-polarization, tribalism, widespread irrationality, a love of conspiracy theories, and our many problems with social media outrage, just to name a few. This does not excuse what they did, but it does make it a bit easier to understand why they did what they did and why they chose the particular means with which they implemented their strategy.
The fake news and added division sparked by the Russian trolls should not be excused or condoned; it should be condemned. I suspect that many U.S. administrations would have treated their efforts as a step toward war. I'm not suggesting they necessarily would have led to war, but I think that almost every administration prior to this one would have taken them far more seriously and responded in a very different way.
But here's what I really want to say: as bad as the Russian trolling was (and almost certainly still is), I am having a difficult time not concluding that the many moronic Americans who uncritically shared the trolls' content across social media were even worse. The Russian trolls and their spymasters were at least doing something they likely perceived as being in their national interest. The Americans who shared this crap were simply duped and ended up amplifying the Russians' efforts to the detriment of our country. They did not critically examine this information because it fit their agenda. Perhaps they didn't even care whether any of it was true.
The most depressing part is that it seems like Russia may have found the sort of weaknesses we probably can't fix. At least, they are the sort of weaknesses we can't fix quickly. And that likely means we will continue to be vulnerable to a host of strategies designed to exploit them. Now is one of those times when we desperately need reason, freethought, skepticism, and critical thinking. Without working to cultivate these things, we will continue to be our own worst enemy.