I love Jessica Simpson's hair, so I've definitely paid attention to it!
She keeps her hair should length or shorter, and the rest of it are extensions. I seems to me that her extensions are wavy, because in every picture where she has wavy hair, her natural hair is straight, but the bottom/under layer of her hair is wavy. That doesn't answer your question, I know, but I wanted to point that out! IDK what it means though.. that she makes sure her extensions are wavy hair?
I purchased a triple-barrel curling iron (Vidal Sassoon from Ulta) to get waves like this.. and I was so horrible disappointed! I had to be very exact with the placement of the iron, and it took for ever, and the end result wasn't close to worth the effort. I have seen a "jumbo" triple barrel curling iron that I think would work much better, since the waves would be so tight. It's made by Revlon, and is currently on sale at Ulta,
here.