No, because the impact is not an ideal collision. There is material ejected, which influences how much the momentum of the body changes. We have a rough idea of how much the velocity should change, but it’s still unknown how much, like on the order of tens of percent compared to theoretical.

If we underestimate how much of an effect an impactor has, we may expend far more resources or take more risks than are necessary to avoid a collision with Earth. Overestimate and we either get hit, or we have to send a second mission which may require an even larger deflection than the original mission (due to a shorter amount of time for the velocity variations to add up to a planetary miss).