J.W.
It's soothing for her, it's a comfort, it helps her center herself. Now with that said, sucking one's bottom lip is better than running around with a pacifier in her mouth or having a solid object like her thumb or a finger there. Lip sucking won't push her teeth backwards. Talk with your dentist. Find another source of centering or comfort for her, like a favorite blanket, a teddy bear to snuggle. It's okay, really. It's a phase and she will travel thru it to the otherside. When she's outside and her wet lower lip gets a little chafed, she'll stop sucking on it.