I agree with your mother. The whole advantage to an outside wedding is it is already beautiful. I had mine outside just for the fact that I wouldn't have to spend any extra money on decorations. We got married in front of a fountain, so we did have a floral arrangement fixed inside the fountain. I would keep it simple, maybe do a couple of pillars with a floral arrangement on them at the alter and petals down the aisle. Other than that, I don't think the extra stuff is necessary.