OK, this one is tough to prove or disprove with any sort of scientific accuracy, but the consensus is that being defecated on by anyone or anything is a bad thing. For one thing, pigeon poop is smelly. And second, while not toxic in small amounts, it can cause serious, life-threatening fungal infections if inhaled in significant quantities, according to the Illinois Department of Public Health.
If that doesn't convince you, consider that pigeons also carry parasites, which may be transmitted to humans through contact with their droppings. When you counter this with the scarcity of evidence to support claims of post-pigeon poop good fortune, it becomes clear that you should avoid standing below a flock of these birds around mealtime.