* Lack of social acceptance: If you're thinking about popularity as being well-liked and accepted by others, the opposite would be unpopular, ostracized, or rejected.
* Lack of widespread recognition: If you're thinking about popularity as being widely known and recognized, the opposite would be obscure, unknown, or anonymous.
* Lack of influence: If you're thinking about popularity as having influence over others, the opposite would be powerless, insignificant, or irrelevant.
Ultimately, the "opposite" of popular depends on the specific context and what aspect of popularity you're referring to.