In the US, nudity is not illegal in national parks-ish.
Things to consider:
1. National Park Rangers generally enforce state and local laws and ordinances.
2. National Parks are frequently patrolled by state and local law enforcement who enforce state and local laws and ordinances.
Don't even think about being nude in a National Park in Arkansas.
Be cautious and careful in Missouri.
Conversely, feel free to be nude in National Parks in Oregon and Vermont.
You'll probably have no issues with nudity in National Parks in California even on high traffic trails as long as you're a couple miles out.
Yes, you can be nude in National Parks, mainly in the western states, as long as you are away from the general public and no textiles are around. You can also hike nude and camp nude in the western states in what is called the BLM--open range areas as long as you are not visible by the general public. I have even camped and hiked nude at the Grand Canyon for 30 days and had the same Ranger drop by to visit and chat several times. The key is being not visible by the general public.
If it was legal? Rather a moot point isn't it? If it was legal would we even be having this discussion? Clothes are over rated, not necessary in most situations. To keep warm, mostly. Think about it. What activities do you do that you need clothes for?