if you’ve never spoken to the girl, then telling her is a giant, creepy no-no.
BUT! if you already have an open dialogue with her and you feel comfortable speaking to each other, then I would go ahead and tell her you’ve have a few dreams about her. Obviously, she will ask what they were about, and at that point, if she doesn’t seem freaked out, and she seems interested, then I think it’s fine to tell her they were a bit naughty. Just don’t use the word intercourse. For some reason, that word comes off as creepy if you haven’t actually slept with the person yet.
As well, the way you approach this will be different depending on the age of the girl. How old is she? If she’s older than 20, than telling her you had a naughty dream shouldn’t be that big of a deal, younger than 20, maybe keep it a bit more PG-13 when trying to explain it…
And yes, please, let us know how and if it goes!