Whilst it is an appropriate use of a nuclear power plant, I'm sure the mass of a nuclear powered probe would have increased the costs and complexity of the launch and landing whilst decreasing the science payload.
It's not a "nuclear power plant," it's a radioisotope thermal generator (RTG). For this particular mission, a RTG would have had a very similar mass to the solar power system used (12 kg for the solar power system, ~12 kg for a 20-30W RTG).
Had a nuclear plant been installed the probe would have had a guaranteed end of life, where as the panels afford the craft the possibility of functioning indefinitely.
The probe will be destroyed as the comet nears the sun, within a year.