Some might say that given this character's relatively dark aesthetic even before being possessed by a demon, that there wasn't much that needed to change. But the subtle changes are there. The stubble, the messier hair, the more formal clothing, it's all very much out of character for Jace. Jace is already a pretty dark character to be sure but he is still basically just a boy, barely into his 20s. Before the possession, all of his features and style points to his youth (read: innocence) the demon corruption pushes him farther into adulthood hence the facial hair, longer locks and crisp suits. Still hot though.
Clary: Simon, how could someone as smart and perceptive as you not realize the person sitting right there is in love with you? Simon: I guarantee you I'm not the only smart, perceptive person to make that mistake.