I think this is more a cultural issue than a Catholic issue. In the Western world, at least in North America, breast-feeding mothers in the Church tend to be more acceptable. They do that in malls and on buses. In that culture, breastfeeding is respected and honoured, and is not viewed as disrespectful to the Lord or the liturgy as long as the woman takes care to cover herself.
However, in other cultures, breast-feeding tends to evoke quite a different sentiment. While ideally we can say simplistically that people just need to learn, we should approach this with sensitivity and understanding. Pastorally we should help people to understand that breastfeeding is not offensive, while at the same time, could speak with those who need breast-feeding to see if it can be done in a way that may reduce the tension it may cause to others who are not ready to accept it.
Of course, if the woman does not cover herself appropriately, or draws undue attention to herself, that would be unacceptable and must be addressed pastorally.