For the many people that answered that seemed to misunderstand what you meant by 'garments' - Mormons that have been through the temple wear temple garments, which replace their underwear. Our temple garments are very sacred to us and represent promises we have made to Heavenly Father. They are worn under the rest of our clothing. So, if a woman is wearing her bra over her temple garment under the rest of her clothes, you would never know.
To answer the question, it is a personal decision. I do, because I am fairly big busted and bras have to be tight to support me, so they irritate my very senstive skin (I have bad skin allergies and am always getting rashes and hives, so my skin is hyper sensitive all the time). I keep my bras over my garments, and the material of the garment protects my skin from irritation. Not all LDS women choose to do this.