When I realised she was a mouthpiece for the writers' opinions and therefore wasn't allowed to be funny.
In comedy characters are usually funny due to some exaggerated flaw. They can be stupid, cruel, arrogant, aggressive, naive, jealous, prideful, etc. But since Lisa is more or less designed to always be "correct" in her opinions, then having critical flaws would make it harder to accept she's right.
TL,DR: The writers don't want the comedy character to funny because they want her to be propaganda.