"A collection of insults 40" joke

Not only rude, but ugly too.

Not playing with / dealing from a full deck (-- not even in the game).

Not running on full thrusters.

Not shooting pool on a level table.

Not the brightest bulb on the Christmas tree / light in the harbor.

Not the full quid.

Not the same since they took him off his medication.

Not the sharpest knife in the drawer / tool in the shed.

Not Turing equivalent.

Not wrapped too tight.

Nothing between the stethoscopes.

Nothing on her radar.

Numb as a post / pounded thumb.

Number' n a hake. (New England expression; a notoriously stupid fish.)

Nutty as a fruitcake.