Does playing by plucking rose petals and repeating "I will succeed, I will not succeed" with the intention of knowing exam results fall under the category of divination, and is imitating this act jokingly considered an act of disbelief (kufr)?

This action is impermissible and falls under the category of blameworthy tatayyur (superstitious belief in omens). It is forbidden, even if done in jest. If the doer believes that it inherently causes good or wards off evil, it becomes major shirk (definition) (polytheism). Therefore, one must seek forgiveness for this action.