Mucocele are rarely seen in the:

The mucous retention phenomenon is restricted almost entirely to the lower lip, seldom found on the upper lip, while accessory salivary gland neoplasms of the lips are almost universally found on the upper lip and only rarely on the lower lip. This could imply that trauma plays no role in the development of salivary gland tumours in this location. Reference: Shafer’s Textbook of ORAL PATHOLOGY Eighth Edition page no 90