Frankly, that's bull. And I can say that with confidence because I've watched the princess movies at least five time each in the past year. (I know, you're all jealous.)
For starters, none of the princesses seem concerned about their looks. Are they pretty? Yes, but then so are the female leads of every other movie. They're pretty just because they are, and they're indifferent to it (Belle, Tiana,) it makes them the target of jealousy (Snow White, Cinderella,) or it's irrelevant (Mulan, Pocahontas.)
Now, some of the earlier princesses do dream of romance. Snow White has a duet with the prince who will ultimately give her "love's first kiss" right before the Queen orders her death, but she doesn't look to him for help. In fact, there's no indication she gave him much thought other than as a distraction from her chores. Cinderella tells the mice she dreamed of romance the night before, but she doesn't expect this to be her reality. It's just a dream, and she says so. When the invitation to the ball arrives, all she wants is a night off from her drudgery at a party (the life she would have had, incidentally, if her stepmother hadn't squandered her inheritance.) The prince pursues her at the ball, and when she finds out the man she spent the evening with is the heir, she is as shocked as anyone. Aurora (Sleeping Beauty) also dreams of romance, and pretends, with the animals, that she is dancing with the lover she also met "once upon a dream." When an actual man enters the fantasy, she allows herself to be romanced, but here's the twist: unbeknownst to her, this man is already her betrothed. She doesn't know he's a prince, and when she's informed she must return to her royal life and wed the prince she is promised to, she weeps for the man she already fell in love with. If she really had been romanced by a commoner in the forest, her story would have ended tragically. It was only coincidence that her love was also her prince.
But in later years, romance drops out of the picture. The princesses sacrifice themselves for family (Belle, Pocahontas, and Mulan.) The romance there comes in the traditional way, slowly, over time, by getting to know someone. (And in Pocahontas's case, it doesn't even work out.) Two princesses, Jasmine and Merida, reject romance outright. Jasmine eventually succumbs to the pursuit of a determined suitor, but only after he proves himself worthy, and Merida manages to stay single. And Tiana just wants her restaurant. It's only after she saves the prince herself, repeatedly, that she starts to fall for him, but even after they wed, she rejects the royal life in favor of her own dream.
I could go on, but seriously, watch the movies yourself. Some of the older ones are products of their time, but the princesses all have virtues besides looks, and these are shown to be the ones of real value.
My daughter plays princess all the time. She watches the movies, then acts out scenes. Never once has this involved "looking pretty" or romancing a prince. When she plays Cinderella she washes the floor and tries on my shoes. When she plays Ariel, she swims in the sea. When she plays Jasmine, she flies around the world on a magic carpet. And when she plays Mulan, she fights with a sword.
So basically, moms, chill out. Your daughter's princess obsession doesn't mean she'll grow up insipid and useless any more than my obsession with astronauts as a child made me a world-class athlete with an advanced science degree.