I understand why you are uncomfortable, society has been sexualizing breasts since before you were born. You, and many others have the sexual function of breasts in the forefront of your minds(thank you media) rather than their primary function, which is to nourish babies. The problem with that mentality, and with expressing your thoughts to suppress a woman's right to nurse her child, and that child's right to eat, is that it continues the de-normalization of breastfeeding.
Many people nowadays rarely, if ever, see women nourishing their babies with their body. Those people become parents or grandparents or aunts or child care providers and have a disconnect with (God's or) nature's design and choose to feed formula, which is proven to raise health risks. The increase in health problems as well as the enormous environmental impact of formula use affects everyone. That disconnect with nature's design can also lead women to not trusting their bodies' natural capabilities and intuition when it comes to health diagnosis, childbirth, taking control of health issues etc. It has been proven that the more a woman sees breastfeeding and hears positivity surrounding breastfeeding, the higher chance she has at breastfeeding her children and the longer that they will be able to nurse.
The health of children, the environment, economy, and society will be greatly improved when the majority of mothers breastfeed.
So try this. Instead of expressing discomfort with a mom nursing(covered or not) (verbally or with body language or after the fact gossiping), smile, nod, imagine your own babies, nieces, nephews, imagine love and support and realize that breasts were made to feed babies and mammals were designed (by God or evolution whichever you believe) to consume their own mother's milk. And parenting can be hard enough without criticism.