I have moderate-severe psoriasis. Can I have plastic surgery for my sagging eyelids?

As long as the psoriasis doesn't involve the skin of the eyelids, I don't see that it should limit you from seeking treatment.  Be sure to discuss your goals and concerns with your board certified plastic surgeon and consider the pros and the cons. Be certain your surgeon is properly certified.  I've attached a link where you can start your search. 

Hello and thank you for your question.  The
best advice you can receive is from an in-person consultation.  As long as your psoriasis is isolated to other parts of your body, and you can obtain medical clearance from your medical physician, then you could safely proceed with surgery.   Make sure you specifically look
at before and after pictures of real patients who have had this surgery
performed by your surgeon and evaluate their results.   The most
important aspect is to find a surgeon you are comfortable with. I recommend
that you seek consultation with a qualified board-certified plastic surgeon who
can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon

Psoriasis on the body will not affect the outcome of surgery on the eyelids.  You should heal fine.