It depends on how you define serious.
Was it unique to Russia? No.
Was it unique to 2016? No.
Do similar efforts go on every day in every walk of life from individual, commercial, criminal and governmental sources? Yes.
Did it alter a single vote? Unknown with certainty but it hasn't been demonstrated.
Were their efforts more sophisticated than the movie that got blamed for Benghazi? Not by much.
Did Facebook profit from it? Yes. Both directly and from the huffing and puffing which ensued.
Is it desirable that this sort of thing happened? No.
So I guess I consider it serious based on the last bullet point an to an extent the penultimate one as well.
What I find more serious is that although it was rife with ineptitude and clearly inconsequential, that it has turned into a historical moment to some people. Worse yet, it has turned into that because we were told that it should and nothing more. That was in turn used to try to unseat an elected President. I would not be shocked if similar efforts, still unpublicized, had been made by Finland and Peru. Trump's homeland sec leader was named Kirsten which sounds Finnish to me. Investigate!