Certainly, NASCAR drivers are the best on the planet at what they do: driving very large cars around in circles, all year long. Do that professionally, day in day out for years, and you have a cadre of folks who are highly skilled at doing just that. This is why NASCAR racing is so "hard" (as mentioned above) for the uninitiated. Obviously, no one is better at driving NASCAR cars than a seasoned NASCAR driver.
However, if you put a top NASCAR driver in an F1 car, they'd probably kill themselves and most other drivers near them on the first lap. It's clearly not even in the same universe. It's like comparing Roger Federer to your local high school tennis champ!
A 40+ year old, slightly overweight, successful NASCAR driver can <span style="text-decoration: underline">not</span> find success in an F1 car, unless it's in a parking lot. He'd have the same chance at winning in F1 as a 40 year old tennis player would have winning Wimbledon (e.g., zero)! When's the last time there was a successful 40+ year old F1 driver?
ANY F1 driver could segue relatively well into NASCAR, or practically any other race car. Sure, it would take awhile to acclimatize, but the F1 driver would do it well. F1 drivers are younger, in better physical condition, have exceptional reflexes and hand-eye coordination, and did I mention <span style="font-style: italic">younger</span>?
No comparison.
At all.
But darn, there's a good reason why NASCAR is so successful: it's FUN!