This is a good question, and hard to answer definitively. As far as I can tell, we've had approximately 60 incidents in the last 30 years which seem to be considered mass shootings, about half of which occurred in the last ten years. There are a lot of different sorts of incidents mixed together there, though; workplace shootings, racially motivated attacks, school massacres, etc.

But my point stands either way; even if rampages are increasing, overall levels of violence are decreasing.