My impression is that it depends on where you are camping. For example we left Florida, proceeded thru 21 states and and over 30 camp spots. All the way up to Idaho, back thru MT and the Dakotas and then southback down. 10,000 miles and never got an advanced reservation. Our tactic is to locate a camp a few hours ahead when we are ready to stop, call then for a spot and we got a place every time except once and that was right outside Yellowstone. And there a second call to another place got a spot.
So no in the center, SE and most western free states there did not seem to be a campground glut. However in states where they close the state parks and limit camping ro every other spot, thus eliminating more than half the camping spots then I believe there is an overcrowding situation. This is mainly the NE states and maybe the left coast. Plus NM is an outlier with a major campground lockdown