Mitsubishi measured the thickness of the boundary layer at the trailing edge of the roof of the Lancer Evo at 30 mm (
source). Consequently, they sized their VG's to that height - "the optimum height of the VG would be nearly equal to the boundary layer thickness."
I can't tell for sure by looking at the Prius C, but I don't think those bumps are anywhere near that height from the surrounding surface. Yet we can be pretty sure that owing to side mirror turbulence, the boundary layer around that part of the car is not going to be very thin.
So, still skeptical.