The mullet type of haircut, long associated with United States redneck culture, appears short on top, long (shoulder-length or more) at the back, with the ears exposed. The Beastie Boys purportedly coined this word-usage.
Mullet in slang refers to a person who mindlessly follows a fad, a trend, or a leader (similar to the older slang term "lemming"). The phrase "stunned mullet" expresses the absence of vivacity and discernment of such a person. The allusion here is most likely to the mullet, a nearshore schooling fish with rather large, blank-staring eyes; the term may also have gained some currency with the popularization of the term mullet for a particular type of haircut (see above) claimed to be favored by the undiscerning and uneducated.
See also: white trash