We do need to be mindful of which trees we hang from.
I know aspens and birches have bark so thin that it's very easy to mar the phloem of the tree. This should be avoided at all costs.
But many species of trees have very thick cork layers of bark, and many species have naturally flaky bark.
The cork layers are nonliving and only serve as protection for the inner living phloem layer. IMO, a little visual marring is really not a concern as long as that marring stays on the outer cork layer and does not penetrate it damaging the phloem.
Also keep in mind that some bark regenerates more quickly than others. It's beneficial to understand your tree species. Pines with flaky bark, for example, will regenerate new bark fairly quickly. A hardwood whose bark mostly splits/spreads to allow the tree's diameter to grow is another issue, and on such a tree, a little visual marring each time someone hangs a hammock in a busy place can turn into more than just a little visual marring over time.