You know how I can tell that it's a sweating problem? Here's how:
Aerith Gainsborough is ALSO wearing sleeveless, and she doesn't have dark underarms, shadows or no shadows! Yuffie, on the other hand, DOES have dark underarms! I saw it everywhere I could find her! Traverse Town, Olympus Coliseum, Hollow Bastion, etc.!
As for Tifa, well, I can sense that she has a sweating problem too because of how she looks and fights. I'm pretty sure that if they put her in Kingdom Hearts, she'd have the same dark underarms as Yuffie. LOL.