Yes. As an egg goes bad, proteins and other chemicals decompose and release volatile molecules like carbon dioxide (CO₂). The shell of an egg is somewhat porous, so these gases can escape, decreasing the mass of the egg. Since the size of the egg doesn’t change (assuming you don’t break it), the density decreases as the egg ages, and it eventually becomes less dense than water. Whether it becomes buoyant exactly when it is spoiled or weeks after, we can’t say. That’s why your nose is so good at smelling that awful smell—so that you don’t eat rotten eggs!