Hello, Alex. Yes, Lemmy refers to himself as a 'damage case' because he has escaped from a mental institution. Doctors and mental health professionals sometimes refer to their patients as 'cases'. Lemy is saying that he is a damaged case, meaning he is insane. A more common English expression with the same meaning is 'damaged goods'. If you call someone 'damaged goods', you are saying that person has a lot of emotional issues. It's a different expression with the same meaning.