1) The poor don't need more food. Obesity is a problem for the poor in America; except for people who are too screwed up to get food stamps (because they don't have an address), food insufficiency is not.
2) Food stamps only imperfectly translate into increased cash income, meaning that the poor will spend . . . more money on food.
3) If the increase in food stamps takes the form of expanded eligibility, rather than larger grants, the administrative issues and public outreach will delay your stimulus until well after it is no longer needed.
4) The limits on the type of goods available to food stamp consumers, and the growing season, mean that some (it's hard to say how much) of the food stamp spending will simply draw down perishable stocks rather than generating new economic activity. Eventually this will probably generate more economic activity, but probably well after your stimulus is needed.
5) The economy doesn't need a food sector more distorted by daft government programs than it already is. If you want to give money to the poor, give it to them. Even if they spend it all on drugs, it will hardly be much worse than spending it all on increasing their already astronomical obesity rates.