You ask a good question, and not one that we get asked often. We do often get asked about what is appropriate burial clothing for adults, and the answer is that you should always chose something that best reflects the individual.
For stillborn babies, this would apply as well, except the parents must decide what clothing captures the essence of the life not lived and what brings them peace and comfort. For example, depending on the religious background of the parents, they may feel they want their child dressed in a christening gown. However, parents can choose any clothing they wish.