Nominative vs Dative
The sentence Meinem Mädchen ist nicht schlecht - surely 'my girl' is the subject of the sentence so should be nominative not dative - this would make it Mein Mädchen (or am I missing something here?)
Yes, you are, and so are words that are IMPLIED in the sentence. If it were written as "Bei meinem Mädchen ist es nicht schlecht" you can see that the girl is NOT the subject of the sentence but rather the indirect object of IT (the state of being not bad). You see it all the time in the phrase "Wie geht es Ihnen?" where the "bei" is left out.