Why do strangers feel the need to passive-aggressively comment, through my baby, on my abilities as a mum?

I used to have a friend who could win awards for passive-aggression. Her signature style was to substitute Irish words in passive-aggressive sentences in a bid to appear less threatening. One morning, for instance, she opened the communal fridge and, noticing there was no milk left, said out loud in my direction, “Stefanie’s the only one who needs bainne, I suppose.” She’d also pass comments of disapproval about people’s outfits by saying things like “Gosh, that’s a very short gúna, isn’t it?”