There are plenty of indications that Kim Jong Un is indeed preparing his daughter as the heir apparent in a long-term effort to prevent a turbulent succession in North Korea. The recent electoral reform, her public appearances at a very young age, and honorific references to her show that he is seeking to prepare the heavily patriarchal and conservative nation to legitimize her succession.
Despite Kim Ju-ae being constantly in the public eye nowadays, this doesn't mean that she will necessarily succeed Kim Jong Un. Given the male-centered power structure in the country, North Korean high-ranking officials would feel uncomfortable about a female leader. It's likely that she's just playing a pacemaker role for a so-far unknown male successor while softening the atmosphere around her father.