Is "I" skipped at the beginning of the sentence?
Native speaker’s answer
Rebecca
Billie Eilish is actually referring not referring to herself here. She is singing to a "tough guy," so the actual sentence can be read as "[you] think you're so criminal. The clue is that the pronoun used in the previous line is "you're." If she was referring to herself, she would have indicated a change in pronoun after that. She is stating that the "tough guy" being sung about thinks of himself as "criminal" (bad).