The insulting capability of "smooth move exlax" runs the gamut from a mild put down (as to an friend who just said or did something reasonably stupid) to a scathing remark on the inane actions of a worthy idiot.
To a submoron who realizes he was about to miss his freeway exit and proceeds to cut across three lanes of oncoming traffic, causing angry motorists to brake and nearly crash: "SMOOTH MOVE, EXLAX!"
Exlax is a common laxative which causes "smooth movements" of the bowels so to speak.
By using this statement, you are comparing a persons action to that of a laxative.
"Smooth move exlax"