Not recommended — #26 of 31 Viscose ranks #26 of 31 fabrics for warmth without weight. It is a poor fabric for this. Ranked on warmth relative to weight, with a packability bonus. The top pick is Cashmere.
Viscose measures 70/100 for breathability, 13% moisture regain, 25/100 warmth, at 150 gsm. Warmth-to-weight is the metric: 25/100 warmth at 150 gsm is unremarkable for packable insulation.