not that i think Jokerman is a man who needs other people to speak for him, but...
in fact, the joke was a little more complex than just being opposite. see, the whole joke builds on smart and dumb being opposites. so don't think smartass-dumbass. look through your mind's eye, and envision, if you would, the word "dumb-ass" spelled just like that, in two "parts". what Jokerman so shrewdly did was to change the first part of the word with the opposite, whilst leaving the latter part of the word untouched as "ass" is a demeaning word in itself and doesn't need a spin on it.
i can see where you had problems, though.