It's way too heavy. An RTG needs a lot of metal to work, encapsulation of the plutonium, radiators, and it needs size - there has to be a temperature differential for it to work, and for that you need a certain amount of distance to dissipate the heat across.
And it's way too inefficient. The RTG used on the Voyager Probes produced about 2400 Watts of thermal power, which was enough for 157W of electricity. The total weight of the device was 37.7 kg. This Parrot drone consumes 14.5 W when hovering, so even if RTGs scaled in a linear manner (which is optimistic), a large enough unit would be larger than the payload capacity.
The hydrogen tank in the structural members carries 120g of fuel. You could extend the longevity of the thing enormously by fitting a secondary fuel tank as part of that 1kg payload you're allowed.