What agency has been criticized for not adequately protecting consumers against faulty sunscreen labeling?

The Food and Drug Administration (FDA)

The FDA is responsible for reviewing over-the-counter (OTC) products used for health purposes, to ensure that the ingredients used are "generally recognized as safe and effective" (or GRASE in industry jargon).  In recent years, the FDA also was asked to deal with various labels and claims placed on sunscreen and sunblock products.