Considering that a F-22's RCS is roughly the size of a marble, and it is expected to fight similarly stealthy aircraft I'd imagine that you're very much under estimating the ability of a modern jet to detect your swarm of tiny cubes. Next, how exactly do your cubes stay up in the air for long periods of time? You'd need a great deal of energy in order to fly at the altitudes which an F-22 would be flying at. And presumably to loiter up there for long periods of time. Suddenly your cheap tiny swarm is expensive, large, and unweildly.