Yes, it is! One of the major ingredients in mayo is any kind of acid that helps the egg whites mix with the oil. This could be vinegar, lemon juice or just plain citric acid. But all these acidic ingredients are very, very rich in vitamin C which is a bleaching agent. Vitamin C helps remove dark spots, pigmentation and tan. It also helps lighten skin tone. So yes, mayonnaise is fantastic for dark spots thanks to it's acidic nature.