If you're looking for manga similar to No Competition, you might like these titles. All recommendations are made by Anime-Planet users like you!
Lizzy has taken second place to her beautiful sister, Maria, for too long. One day, Maria dies in a traffic accident and Lizzy decides to adopt her sister's orphaned son, Ben, whose father is unknown. Since then, she has been living a happy life as a single mother. But one day Prince Enrico comes to claim Ben, saying his deceased brother is Ben's father. Ben is the only heir to the throne of his principality. "I have nothing to live for but Ben!" Lizzy firmly refuses Prince Enrico's claim...
If you like No Competition you may like Royally Bedded, Regally Wedded.
This is because they are both Harlequin romances in which the female main character feels inferior to her sister who is very beautiful and a model.
Both female main characters are not bad looking, but have insecurities based on features they have that they would not define as traditional beauty. An example of this is freckles, which both are mentioned to have, though the freckles are drawn more visibly in No competition.