Can I decoupage a sheer curtain onto the interior of a door window?

If your curtain is sheer you aren't going to get much privacy. I would try some privacy film. It comes in many designs-some more solid than others that will offer you more privacy than a sheer curtain and some more decorative and open that will let more light in. Amazon has many rolls/designs of decorative privacy film for reasonable prices. I think using privacy film would be better than decoupaging a curtain to your window.

All I can say is”ditto” the bonus for me living in a colder state....the privacy film has helped the frost not stick to the windows

https://www.hometalk.com/diy/decorate/christmas/christmas-stained-glass-windows-clear-contact-paper-5369366

https://www.hometalk.com/diy/windows-doors/windows/privacy-glass-using-contact-paper-27032870

https://www.hometalk.com/diy/windows-doors/windows/how-to-use-contact-paper-to-make-a-beautiful-privacy-window-27632620

https://www.hometalk.com/diy/windows-doors/windows/vinyl-window-clings-for-privacy-31527465

https://www.hometalk.com/diy/frosted-contact-paper-cut-out-shapes-cute-modern-window-covering-2867991

Instead of a sheer, try window privacy vinyl. It comes in many styles and is applied with water and smoothed out with a squeegee. It cuts to fit and can be changed to something else easily. You can get it at Lowe’s or Walmart.