The OED defines a cretin as "one of a class of dwarfed and specially deformed idiots found in certain valleys of the Alps and elsewhere." Used in a more general sense, a cretin is a fool or someone who behaves foolishly. Harsh! The origins of the word are somewhat refreshing, however. Cretin comes from the French Swiss crestin or creitin, and, more recently, crétin in plain old French, meaning "Christian," as in a respectable human being. Applied to these valley-dwellers, it served as a reminder that those who are not "normal" in manner or appearance still deserve kindness and respect. Then again, it is a shame that a reminder should be required, and I may be giving whoever coined the term a bit too much credit.
As for what The Ramones were getting at...well, give me another month and maybe I'll be able to make out an adjective or two, as well as further cement the mental image of them recording all of their vocals while being violently shaken.
Cretin is pronounced CREET-in (long "e") or CRET-in (short "e"). The latter pronunciation appears to be the standard in British English, while the former is more common among speakers of American English.