Is it permissible to describe those who circumambulate graves as polytheists?

Tawaf (circumambulation) is not permissible around anything other than the Kaaba. It is an act of worship that Allah has only prescribed around His House, as He Almighty says: (and circumambulate the Ancient House). Whoever performs tawaf around shrines and graves intending to draw closer to the occupant of the grave has fallen into major shirk (polytheism). And whoever does so intending to draw closer to Allah through this tawaf has fallen into a reprehensible innovation. As for the ruling on a specific individual who performs this act, it depends on their intention, and whether they can be excused due to ignorance or misinterpretation, or not.