I don't think Kagari is angry that she could have been a romantic interest for Noctis. But as it seems now, Luna has only literally become a love interest. She has no other purpose in the story, as her abilities she shared with Noctis to fight as an equal with him as a legitimate antagonist are nowhere to be seen, and we only see her sitting in a chair or getting beat up.
It raises no confidence that she is literally anything in the story outside of a hold over from those days.
Look forward to burly dudes frolicking in the tent. That holds more appeal that a strong female character.