No, eating healthy should not be mandated by law. This is America, the Land of the Free. No government has the right to tell us what we can or cannot put in our bodies. The government should simply recommend what we should put in our bodies in order for us to live a healthier and longer life as well as not be such a burden on the medical system.
People should make choices about what they eat for themselves. People have different diets for religious reasons, matters of taste, and personal reasons. What one person thinks is healthy may not be to another. Also, it would be highly Fascist to tell someone what to eat. It's a bit like acting like you own someone.