auto racing in general is most definitely a sport. the top levels of open wheel racing are some of the most demanding sports physically and mentally. i forget where i read this, but they recorded the heart rate of an indycar driver over one race. and his average heart rate was higher than marathon runners. its a lot different than something like running where it takes pure cardio strength, its resisting the constant very high g forces, strain on literally every part of your body, and just the pure physical effort required to even turn the wheel and brake in a car with that much cornering and braking grip for 2+ hours. and to be in those conditions, at temperatures well over 100 degrees for hours straight, and having to use every little bit of mental ability and focus you have, because loosing any bit of that focus for a split second means a big mistake or crash, id argue that it is the most overall demanding sport.
some drivers train for hours a day 5+ days a week in the off season, to prepare, and just the act of driving is enough to keep them in shape during the season. so id say that says a lot as to how physical it is to drive something like that.
now that being said. nascar does not take as much physical ability, but it takes quite a bit, certainly more than a park skier doing a park run in competition. nascar drivers are some the most talented athletes in the world. oval racing in the US has a shit ton of talent, and it takes amazing ability to rise to the top, id argue that there is as much talent in nascar as there is in F1. people talk shit on nascar because the fans happen to be redneck. but its more of a sport than the majority of other sports.