In US usage, the derogatory connotations of the term "plastic cowboy" are generally imbedded in usages of the derogatory sense of "cowboy."

A sparkling, gistening, glowing example appeared quite recently, in reports of prisoners' lawyer's complaint against Gitmo representatives, in
Gitmo prosecutors taken to court:

"Are these prosecutors bound by the rules that are binding on all prosecutors everywhere?" MacLean said. "Or are these prosecutors going to be allowed to be cowboys, doing whatever they want?"

I'd expect that the judge(s) and the Gitmo prosecuters knew exactly what he meant, even if the term is less commonly used in that way than was the case a decade or two ago.

John