Is drill baby drill an aphorism?

Yes, one definition of Aphorism is "a statement of principle" making "drill baby drill" used in the 2008 Republican Campaign an aphorism. An aphorism is an original thought, spoken or written in a laconic (concise) and memorable form. (Wiki)

"Drill baby drill" is an oil field term stating the order of business in the oil industry, "drill first then ask questions later" is a good parallel expressing the 'Manifest Destiny" that oil companies lobby in Washington.
It is easy then to see how it became a slogan during the 2008 Republican Campaign.