Original fabric on Fritz Hansen desk chair?  

Do you have an overall photo of the chair? The fabric looks later to me but that is just based on having followed upholstery fabric trends for a few decades, not any encyclopedic knowledge of fabric patterns in general.

Hand sewn seams are a basic upholstery skill, and as such are not really an indicator of original upholstery. Any decent upholsterer knows how to do a blind ladder stitch and it's actually used a lot on conventional furniture, not just curvy mid-century modern pieces.

Sorry it took so long, Spanky.

A side view would help to determine if the chair has a bit too much padding. But just based on the fabric pattern, I'm inclined to say it's not original----but I'm also an expert on these chairs at all, so take that with a grain of salt!