I made this mistake as well initially - but ordinary viscose isn't. It's actually comparable to or even worse than cotton in terms of overall sustainability.
Cotton uses huge quantities of water in its manufacture - but although viscose is less water-intensive, it relies on chemical processing (with all the associated toxins/pollutants that involves). And although it's biodegradable given the right conditions (whereas nylon or polyester aren't), it's derived from wood - so if it isn't sourced responsibly, viscose production can actually be very damaging in terms of both pollution & deforestation.
The types to look out for include
Lenzing Ecovero, Tencel Lyocell, Modal, and Bamboo Viscose. They use closed-loop production (so that the chemicals are fully recycled), much less water, wood from FSC/PEFC-certified forests, and their carbon emissions are up to 50% lower than traditional viscose.
I'm glad that most major companies now supply much more information on sourcing & supply chains. It does help, as it is a bloody minefield out there though ...!!!
Lots of my vintage stuff from the 70s is synthetic & therefore fossil fuel-derived. So technically speaking not in the least an environmentally friendly choice. But durability is also part of the considerations - as is reuse/recycling - & most of those fabrics are bloody indestructible!!! I have clothing that looks unworn after almost 60 years ...