The meaning has changed slightly over time:
1970s: An individual or band who took up punk too late after its mid-70s birth, and so simulated the general look of late punk but little of the original ethos. Most 'plastic punks' were fashion-oriented, in it just because it meant they could finally start wearing something non-hippy. A minority had some of the original punk attitude too - but it was directed as violence against other punks and against the teddy-boy revivalist of the time. Often the latter 'plastic punks' caused a lot of the violence at gigs in '78/'79.
1990s onwards: the term is still being used in the USA and UK, to deride "punk" bands who use the music style to spout mainstream liberal 'politically-correct' attitudes.
1970s: "In the incident Joe Strummer, of The Clash had smacked a member of the mob with his guitar, was arrested and released after it had been established that he wasn't pissed. The band were, however, pissed off by this element at their gigs. Yet again, the result of a plastic punk attitude."
1990s, USA: "...forget those impressions of the East Bay as being a politically-correct plastic punk world. The bands featured here are the underground of the underground."
1990s USA: "Perhaps predicting, predating and being too iconoclastic for plastic punk chic, it's unlikely they are going to be on the cover of The Face quite yet."
1990s, England: "Goldsmiths Tavern is only good for a stabbing or a puking plastic punk."