I think it depends on intent.
If it was laziness ("screw there people, I'm not dragging myself and this baby out of this pew"), social anxiety about being disruptive ("what's the quickest way to quiet the baby") those might reflect poor intentions.
If it was a choice about being deliberately mindful and present for the sermon (if one can be mindful and present while nursing) then I think it's appropriate.
Unfortunately, we don't know her intentions.