Would down-stuffed curtains stop heat loss through windows?
Absolutely. Down is a terrific insulating agent. It would work even better if you seal up the edges of the curtains along the window. A good way to do that is with either magnetic strips or velcro along the curtain seams on each side. That way you can let sun in during the day and seal rhem up at night to prevent heat loss.
I assume your windows are single pane. If you want to go a step further you can conserve even more heat by also installing plastic film insulation on your windows. Kits are available online and at most hardware stores.