I'm really confused now!  In her other series,
Urusei Yatsura, R.T. has Lum calling Ataru:
"Darling no baka!".   So does that mean it's
correct if the female lead does it but for 
everyone else it's wrong?

You're working under a misconception.

It's not syntactically wrong, nor does it make a difference who says it.
It's *inherently* wrong because *Akane* *is* *cute*.

Wow, someone who is correct besides me! ^_^

The three rules of the universe are:

1. Kun-chan is always right
2. You can't accelerate beyond lightspeed.
3. Akane wa totemo kawaii

Close enough.... Although it's actually:

1. Kun-chan is always right
2. Akane wa totemo kawaii
3. Kun-chan is always right

Kun-chan...