From my limited reading about various groups of Native Americans, it seems that many European accounts of this-or-that group being described as cannibals was almost always second-or-third hand, something that came from that group's neighbors rather than something explorers and colonists witnessed directly. Is this accurate from what we know? Was it common for indigenious peoples to accuse their enemies or rivals as cannibals?
Yes, but only when they wanted to prevent the Europeans from visiting them. Native peoples usually heard about the Europeans long before actually meeting them because their guns and manufactured goods usually arrived long before the Europeans themselves.
Those peoples wanted to get these valuable supplies while denying them to their nearby enemies. After all, guns are only an advantage if the enemy does not have them. Thus, whenever they first met Europeans they would try to dissuade them from traveling any further or persuaded them to avoid certain tribes because they were cannibals. Saying "those guys are cannibals," and so asserting their potential to hurt the Europeans, was more effective than simply stating that they were their enemies. Europeans rarely had any issue with trading or allying with two enemy tribes.
Sources:
Robbie Ethridge - From Chicaza to Chickasaw: The European Invasion and the Transformation of the Mississippian World, 1540-1715
Jacob Lee - Masters of the Middle Waters: Indian Nations and Colonial Ambitions Along the Mississippi
Kathleen DuVal - The Native Ground: Indians and Colonists in the Heart of the Continent