i was in Sequoia and Yosemite in August. The rangers said OK to use hammocks in the picnic areas (we were day hiking), and Also ok in back country. Campgrounds run by different managements - call ahead imho for campgrounds.
We talked with several back country hikers who said hammocks are used by many backcountry hikers (when they are below the tree line of course).
Have a great trip!
"Climb the mountains and get their good tidings.
Nature's peace will flow into you as sunshine flows into trees.
The winds will blow their own freshness into you, and the storms their energy,
while cares will drop away from you like the leaves of Autumn."
— John Muir