A Mary Sue is a character who essentially is an idealized projection of the author's self, serving as a sort of fictional wish fulfillment. Mary Sues run rampant in roleplay and are nearly universally hated. They're not great characters and they tend to disrupt other people's fun. Make sure that when you are roleplaying as a paladin that you give your character some flaws to balance their strenghts and make them more believable and relatable! In your next game of Dungeons and Dragons, show your love for justice in this divine Paladin design.