The previous Yonko maintained some standards. It was extremely hard to even see one a while back. Their presence alone is considered a calamity or something worth worldwide attention. Luffy doesn't have that kind of overwhelming presence, nor does he maintain the standards of a typical Yonko. That explains why Lucci doesn't want to acknowledge Luffy. Stussy, however, knows better than to judge a book by its cover. Lucci could learn a thing or two from her.