Why is this dative?? Meinem Mädchen ist nicht schlecht.
Why is 'Mein[em] Mädchen' in the dative case here when it is clearly the subject of the sentence??
German often uses the dative to express how someone feels. This is quite common among the Germanic languages. Likewise, ‘I am cold’ translates as ‘Mir ist kalt’ = ‘It is cold to me’. If you say ‘Ich bin kalt’, it would mean something more like ‘I am coldhearted/unfriendly’.
One way to think of it is that the subject of the sentence is not actively doing something but passively experiencing it. So the girl in your example is not actively being not-bad, but experiencing not-badness.