No, immediately lowering heart rate isn't a goal of exercise. The reason it's a meaningful measure at all is because a lower resting heart rate, not overnight in response to a stimulus, but a permanently lower resting heart rate, is a sign that your overall cardiorespiratory system has become more efficient in terms of how much blood it can deliver per beat, how much oxygen it can deliver per unit of blood, and how much energy can be generated per unit of oxygen in your mitochondria. When those efficiencies improve, fewer beats per minute results in the same level of work done in your cells. Thus, resting heart acts as a proxy measure of aerobic fitness, not a goal in and of itself. All of those are long-term adaptations. Conversely, there are many ways to acutely lower heart rate that are clearly not healthy. Death, obviously, but taking opioids or many other kinds of depressants, not moving ever, sleeping 23 hours a day, will all lower your average heart rate immediately without making you fitter or healthier.