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

Yah yah yah, but what has that got to do with "baka"?
Akane no Baka is correct grammar in its context-

Who's talking about grammar? We're talking about what is right and what 
isn't!

Akane wa irokeganai!
(just kidding, Kun-chan... oh no...)

*slapslapslapslapslapslapslapslapslapslapslapslapslapslapslapslapslapslap*

Kun-chan...