The dewlap is some reflection on your rabbit's weight. My Ebony bun is a stocky mini-lop who has a noticeable dewlap.
My vet told me that rabbits store fat in their dewlap in the same way that camels store water in their humps (kind of a reserve store). Although it shouldn't affect the rabbit's breathing, its a good idea to check the skin underneath tobe sureits not irritated (where the dewlap is less air can get to the skin under it).
Different rabbit breeds have different healthy weights, your vet should be able to let you know if your girl's weight is OK.
Hope this helps