To be honest I think while many digital nomads can be respectful of local culture, our presence often erodes it with demands for Western conveniences. From an environmental standpoint, it can be a very wasteful lifestyle with frequent flights, use of water bottles etc.
You can't really hold on to things to live as sustainably as possible since you're always moving around living an electronic lifestyle.
All said, many people are a lot more conservative and respectful than others, but overall it seems to be a very consumptious lifestyle.
I think with more advances of sharing systems and technology around a shared economy we could really improve the sustainability of this lifestyle