At what point in history dd the Corvette become a redneck car?

Base price of a 2020 Corvette is ~$60,000. Corvettes have never been a "redneck car," they've always been too expensive. The Camaro has long been the stereotypical "redneck car" (base price for a 2020 Camaro is ~$24,000), so much so that another term for a "mullet" is "Camaro cut."

"they look cheap and plasticky"

Corvettes have always had plastic body panels (reinforced with glass or carbon fibers, depending on the year and the specific body panel).

You can get certain older Corvettes on the cheap, but the same can be said for most any make/model of car, especially if you consider $45K to be "cheap." You can get certain old Rolls-Royces, Bentleys, Ferraris, etc., in good condition (fully restored even, in some cases) for that kind of money or even less. In reality though, for people who truly fit the definition of "redneck," even the cheapest new cars are a huge financial burden for them (because they have low-paying manual labor jobs), and they usually buy used vehicles for a couple/few thousand dollars.

$70,000 here in Canada to start.