ditz/ditzy
these terms seem used primarily of women, so you may want more inclusive-appearing alternatives: out to lunch, space ranger, flake, out of it, on another planet, with one's head in the clouds, not all there, missing some marbles, with a mind like a sieve, mindless, brainless wonder, dense, muddleheaded, not bright, half-witted, dull-witted, dim-witted, thick-headed, thick-witted, inane.
