According to eyes on the solar system, Dimorphos is tidally locked to Didymos.
If DART impact direction is perfectly aligned with Dimorphos center of mass, all of the momentum transfers into orbital period shortening, and none of it into disturbing Dimorphos' rotation rate.
As SE - stop firing the good guys correctly pointed out, even if rotation is not changed by impact at first, orbital period is, and it will change rotation rate ultimately.
Was DART planned to hit dead center the most accurately so that most energy slows Dimorphos without changing at first its rotation rate? Or was a ratio of impact energy dedicated to create a bigger rotational perturbation, by purposefully missing the center of mass' bullseye?