it's a comical way to say "keep going" or "push through" or whatever. its supposed to be an encouraging quote and usually serves its purpose, also making that person smile
A sly way of calling a girl as perfect and graceful as a princess when cheering her up. An encouragement to just keep standing proud.
Girl: Ugh, literally nothing is going right today. I fell flat on my face in the audition, and my best friend is not talking to me anymore. I just feel so awful.
You: Oh that's awful! You deserve way better than that. Just Keep Your Head Up Princess, Your Tiara Is Falling