This is a quote ,that has become a known bumper sticker by an unknown author.
It simply means if you are not angry or shocked,then you are overlooking the facts or you are not consciously aware of them.
It was used for example as a title of an article written by John Sweeney.
It starts as follows:
" We're at war. Wages are stagnant. Gasoline prices have doubled since 2001. Pensions are disappearing. Health coverage is slipping out of reach. Parts of New Orleans remain uninhabitable."
Then he uses this quote:
" Really, if you're not outraged by now, you are not paying attention."
So, if you are not upset, angry or shocked by a certain situation or circumstance due to its obvious unpleasantness, then it is a sign of lack of care or attention.