Pearl being elfish implies several possibilities. First, an elf is a mythical or folkloric creature known for causing mischief. They are tricky, and manipulative. Pearl is equally manipulative of her mother's emotions, especially in regards to the scarlet letter.
Another "mystic" aspect of being elfish is the idea that such creature comes from a mysterious place where the behaviors and customs are equally mysterious. As we know, Pear seems to have advanced knowledge of the emotions of people. She tends to push buttons because she has a more active and inquisitive perception than the average person. This leads to yet another aspect of Pearl's elfish nature: Her unusual behavior.
The elfish nature of Pearl also implies that she will behave like a person much older than herself while still inhabiting the body of a small child. Hester would agree with this latter description. Even Hester felt as if Pearl had some power over her than rendered her weakened.
Therefore, Pearl's strange behavior toward her mother and the rest of the villagers earns her the description of "elfish" quite well.