"Ain't Misbehaving" definitely DOES mean one is being "good," whatever that word means! To misbehave is to violate expected moral conduct. Not MIS-behaving is semantically equivalent to behaving (the implication of behaving rightly or correctly being understood, since in fact even a bad actor is behaving.)

Nor is it a correct example of litotes, since the intention is to explicitly state one is not doing something (misbehaving) not exagerrate by understatement, nor to make an assertion more modest. Misbehaviour is defined as "bad" conduct and the phrase is an assurance that one isn't engaging in it. The argument about being dead is moot since the expression's original contect is in the first person singular, and thus an assertion not likely to be offered by someone dead.

The writer could have said something to the effect that his current conduct was not unvirtuous. This is the "modest assertion" type of littes. If the writer had said "My behaviour is not wildly licentious, never fear!" or if the writer had said, "I guess I am not an angel but I am saving my love for you....", the other kind of litotes (emphasis by contrast) would have been created. But saying one isn't misbehaving when one is not misbehaving doesn't qualify IMHO

A