If the IO asked you to sign the pictures and application with your regular signature, then you need to sign the certificate in the same manner. This is what I was told and what I did, as the signature on the picture should match with what you sign on the certificate. I have since applied and received a passport - so obviously no problem.
I never sign my name in cursive and never intend to.
I like that. I did sign with my regular signature on the photos as the IO said. So my certificate will have the same signature ( Cursive in farsi)