Anything sour can potentially aggravate the Pitta dosha in the body. Pitta primarily resident in the small intestine and liver. If someone were already suffering from a significant Pitta imbalance, sour tastes would not be advised.
I have a significant amount of Pitta in my constitution and as a result, can not consume large amounts of citrus, such as drinking a full glass of orange juice, because it overheats my digestive tract (sour stomach, loose BMs).
Lemons and citrus are healthy when taken in the proper amounts for one's own constitution.