Terminal velocity applies to a falling object where gravity balances drag, so not the right term in this case.
I reckon two things apply – energy and inertia
Ass stated, energy of a car depends on velocity, so if the car is doing 25 at the very point of impact, the energy it has (and can impart) is the same at that one instant, no matter if accelerating, constant or decelerating.
Inertia says the struck person will need to be accelerated from 0 to 25mph (or slightly less, as the car will also have the force of the person acting on it, trying to slow it down). In this case, the acceleration of the car would then take effect and decide what happens in the moments following the impact. Energy is still being added for the duration of the impact. So I’d rather be struck at 25mph by a braking car than an accelerating one (if I had to be struck at all – not offering to try it!)
I think that’s about right…