Second Fitting - Hate the Back Muffin Top!

Your dress is beautiful and flatters you so well, first off. Second, it's really just the cut of the dress. If you look at any bride's pictures with a similar back, that's what our skin does. Even if the bride is slender. (As you are) I don't think yours is that severe but you could always use tape to sort of pull the skin away as PP's have said. I don't think any shapewear is necessary, that won't make a difference. Try not to fixate on it regardless because nobody else is going to see your back and think "muffin top" 😅 You look stunning.

Your dress is really gorgeous! Like someone else said, perhaps a veil would help to distract from the back. I tried on a few strapless dresses that did the same thing and I hated it but the sales lady ensured me that no one is as critical on appearance as the bride herself! (maybe she was just trying to make a sale but it made me feel better )

I think the only thing that would fix that would be a higher or drastically lower back.

Honestly, that dress is stunning and more importantly, it's stunning on you!!

It has nothing to do with your body. It's just the shape and cut of the dress. I would talk with the seamstress and see what she thinks.