How old is your daughter?
It shouldn't transmit to your daughter, provided she isn't a newborn or have any broken skin.
Whether it's a cold sore, a burn, a cut, or something else on your nanny's lip, you have every right to request that she not kiss your baby while she has symptoms of something on her lip. It's not your place to ask what that is, but if it's an infected burn, she shouldn't be kissing your baby any more than she should be if it's a cold sore.
Also, there are colds, flu, covid, strep, dozens of upper respiratory viruses that affect kids - she doesn't need to be kissing your baby on the mouth. No one does except her parents and siblings, really. You have the right to request that, too.
I wouldn't worry so much about the belly kisses if she's of an age where it makes her giggle. She's old enough that her skin is thick enough to be a barrier.