Crowded campgrounds during the summer is like what they say about real estate: "Location, location, location".
Used to be that traveling in the spring before school was out and in the fall after school started just about guaranteed that you'd have uncrowded conditions. Not no more. And summer is going to be worse, so I would figure if you're going anywhere fairly popular it's going to be crowded. How crowded is too crowded is up to you to decide. To me a half-full campground is crowded, but you may not feel the same way.
Just like old peoples' undergarments: "Just depends". :)