“Immortal” simply means not dying. You could 206 years old, unable to leave your bed, toothless blind and withered, and be immortal.
The Norse gods, as interpreted by Marvel, are just long-lived aliens.
Yes, in that instance, teeth are immortal. Brushing and flossing are a must for appearance sake. They would have to be immortal since teeth are the weakest link in the body. My grandmother, who died at the age of 96, had all of her teeth and not one cavity! (Supposedly due to high natural fluoride levels in the water of western Oklahoma where she and her family lived.)