Why are some tummy tuck lines so straight while others are like a zig-zag or not even? Is it the surgeon or the person's shape?

You correct on both accounts.  The incision shape is influenced by the surgeon's personal techniques as well as the tension needed to close it properly based on the individual patient.  I encourage you to discuss this concern with your surgeon so you understand what to expect after surgery.

You are quite perceptive.  Some surgeons use a w shaped pattern which I think looks messy.  I use a gentle smile shaped incision that I think heals much more neatly.  It is worth discussing this issue with the surgeon a head of time so that there is agreement on this issue.