Is citric acid is a anti foaming agent?
Citric acid is not a defoaming agent in itself. It is routinely used for pH adjustment in many commercial hand-dishwashing agents.

But it depends on the surfactant in the product. As Ivan tells, anionics will at some point be protonated by acids, and thereby loose their surface activity –and foaming capability. 

Commercial anionic surfactants like ethersulfates will only do so at rather low pH, outside what is relevant for dish-washing products. But if your product is based on saponified oils it’s another story. Many fatty acids are weak acids (pK in the rage of 4-5) and will be significantly protonated at pH even as high as 5-6.

By the way: making good foam is as much an art as a science :-). If you want to dig deeper in the subject, I can recommend “Prof Steven Abbot –Surfactant Science, Principles and practice”. It is freely available on the web.