A strap-like lower lip is a characteristic feature of Class 2 division 1 malocclusion because this condition involves a marked protrusion of the upper incisors and a retruded mandible. The forward positioning of the maxillary teeth and the retrognathic mandible cause the lower lip to stretch and appear thin and tight, resembling a strap. This compensatory lip posture attempts to seal the mouth but results in the notable strap-like appearance. Such clinical presentation is distinct for Class 2 division 1 compared to other malocclusion types.

Reference: Contemporary Orthodontics, 6th Edition.