It's offhandedly calculated. The random people don't mean anything to her. So, she only bothers to act more like a human in front of those humans she really interacts with. Most people don't know of parasytes, so they wouldn't initially think a woman carrying her baby in a wrong way is a real monster. They might wonder about it, some might say something, a few might eventually even report her to the authorities, but in the end that only happens rarely and mostly if they see her doing it more than once. Let's not forget it takes her effort to act even remotely like a human and trying to do it continuously might distract her from more important stuff.