Other hyponyms of simple include
airhead,
dimwit,
doofus,
half-wit,
nitwit,
dingbat,
boob,
booby,
dope,
dumbbell,
dummy,
pinhead,
fool,
muggins,
saphead,
tomfool,
changeling,
cretin,
half-wit,
idiot,
imbecile,
moron,
retard,
idiot savant,
nebbech,
nebbish,
nincompoop,
ninny,
poop,
forgetful person,
scatterbrain,
schlemiel,
shlemiel,
schlep,
schlepper,
shlep,
shlepper,
schnook,
shnook,
sheep,
space cadet,
lame,
square,
dolt,
dullard,
pillock,
poor fish,
pudden-head,
pudding head,
stupe,
stupid,
stupid person,
subnormal,
twerp,
twirp,
twit.