Butterflies can symbolically represent the blossoming of a person going through the larvae > pupa > butterfly stages. Blossoming is of course also linked to flowers, the name of Robin's fruit and also one of her biggest passions. Oda has drawn the butterfly tattoo so that we only see half of it. This may be Oda's way of saying that Robin is still only half blossomed, and that she still has plenty of developing left to undergo.