Yellowstone is good if you actually hike and do the physical things. If you only do the geyser paths and driving it is hard to be away from a crowd. And so many idiots trying to get close to wild animals (╯°□°）╯︵ ┻━┻

Honestly, that's the case for most of the more famous national parks but Yellowstone and Grand Canyon are the ones that get a surplus due to their worldwide fame.

That said I still count it as a place every American should visit and think it's worth a visit. If you really want to reduce the other people, visit in winter but some things may not be available but some other things are.