Collins Dictionary:

The lights are on, but nobody’s home means that although someone seems to be normal or satisfactory, they are in fact, very stupid or useless.

e.g., According to Harrington, many projects are not properly thought through. You get the feeling that the lights are on, but nobody’s home.

Hello, Pete? Earth to Pete! He must be daydreaming again—the lights are on, but nobody’s home.

I kept asking the suspect questions about what happened, and 

he just kept staring at me blankly. 

The lights are on, but nobody’s home, you know what I mean?

Tradução: alguém parece ser esperto, atento, mas na verdade é estúpido ou não tem serventia.