The joke term stems from traditional turntables (Technics 1200's etc) being referred to as "The Wheels of Steel"
The extra time and practice it takes to master the art using the "wheels of steel" vs the "wheels of plastic" tends to create more skilled DJs. The fact that DJs on the "Wheels of plastic hardly ever end up learning the art of Record control also means they usually don't learn the more advanced scratches and turntable techniques associated with Traditional Vinyl DJ Equipment. So the term also makes reference to the plastic / fake nature of the DJ using the wheels of plastic.
Most of the good DJs that actually do use Wheels of Plastic started by first learning traditional turntables and often have the skills associated with that learning. Many others don't and the term really is used more to describe a weak CD DJ and their choice of equipment.
"Damn, dude did it small on the wheels of plastic."
"He was up there in the DJ booth on the wheels of plastic faking like he was doing something, jumping up and down waving his hands and making it look ultra hard to turn the knobs on the mixer what a douchbag"
"The two DJs rolled in to the nightclub's little makeshift DJ competition with some Techs (Technics Vinyl Turntables) and anihalated all the phony wheel of plastic DJs that showed up to compete."
"the Wheel of plastic DJs all lowered their heads in shame when they saw DJ Craze kill it on vinyl turntables."