Airhead

20 clues
???
Dolt
Dope
Fool
Ditz
Ninny
Dunce
Doofus
Numskull
No-brain
Numbskull
Total ditz
Ditzy type
Space cadet
Scatterbrain
Featherbrain
No deep thinker
Featherbrain kin
One with an empty attic
Someone with nothing between the ears?