It's been repeatedly stated that Tiffany/Lash is a kind of Lasciel. She isn't part mortal at all, she's knowledge that is itself divine and scraps of stolen divinity shaped like a mortal woman.Like Harry said "Lady, you're no Lasciel."
Dresden has actually looked upon a Fallen and gotten bounced onscreen; he knows the difference
Tiffany wasn't even made with human parts, but rather an impossibly perfect idol designed to facilitate her worship as a sort of god.