Yes, smelling someone's underwear without their consent or in a persistent and uncontrollable manner can be considered an obsession and is generally regarded as inappropriate and intrusive behavior. This behavior could indicate a form of paraphilia or fetishism, which are types of psychological disorders characterized by intense and recurrent sexual arousal to unusual objects, situations, or body parts.
If someone is experiencing this kind of obsession, it's important for them to seek professional help from a mental health expert or therapist. Such behaviors can cause distress to the person experiencing the obsession and can also be harmful and distressing to others involved. A mental health professional can provide guidance and support to help the individual address and manage these impulses in a healthy and respectful way.