Any surgery leaves scars. Gravity also continues to work against us as we age and things will sag over time. The quickest answer to your question is that your wife needs to wait to consider any surgery until the baby is born and at least 3 months has passed since weaning. Surgery should not be done on a breast that is making milk because problems can come from cut milk ducts in a lactating breast.
Also, breasts change during pregnancy and after weaning, so the asymmetry your wife has now may be minimized when she gets her body back, so to speak. A woman's body changes during pregnancy and those changes do not always make a woman feel attractive, something like asymmetry may make it worse, but ultimately, she is in the process of something very important and that has some beautiful aspects to it too.