OK, while the term "all babies are beautiful" gets thrown around a lot, let's be honest: there have been times when you've seen an infant and thought, "Ew."
Look, beauty is in the eye of the beholder, and sometimes those eyes are sideways, checking out that disgusting garter belt thing you put on your bald daughter's head to make her look more feminine. Not everyone is going to think your son's weird ears are endearing, or recognize the charm in how your wrinkly little girl is the spitting image of her elderly grandfather. Also, if it comes as a total shock to you in 2013 that people on the Internet are capable of being complete assholes, then you are a pitiable rube.
That being said, making fun of people for how they look is certainly not nice. So it was no surprise that when a private Facebook group—initially intended to sell and trade toddlers' clothing, but turned into a space for people to get really real about the physical appearance of strangers' kids—was discovered, there was outrage at the meanness of it all. Because everybody expects mothers to be kind and nurturing and benevolent, even though there's plenty of evidence to the contrary.
Naturally, a mother is protective of her child, and doesn't want her kid to be called ugly. But at the same time, it's also probably because she thinks her child is beautiful, and lends a certain amount of importance to the issue. (Don't we all, really?)