First, as Dwarfenstein says, many shields were rectangular. (EDIT: the comment I was referring to is gone. Basically it just described ancient Roman shields, and made the point that rectangular shields were common in some eras.) There were also kite shields, which tapered to a point, and heater shields, like a kite shield but with corners on top to catch attacks with.

But, to answer your question, no: a circular shield isn't appreciably harder to make than a square one. In fact, most large rectangular shields were curved to fit around the wielder's body, requiring extra work to shape, while a round shield can be flat (it doesn't have to be, but it can). And in any case, all of these designs were well within the ability of medieval armorers to create, so weight vs. effectiveness is a larger concern than ease of construction.