In Gypsies Tramps and Thieves, what does Cher mean by "Dr Good"?

It sounds like a brand of quack medicine that was promised to have many theutic benefits but failed to deliver on those promises. Once the buyers realized that they had been duped, the people who sold them were long gone. Those were especially popular in the late 1800s and through much of the early 1900s. That's probably where the term "snake oil salesman" came from.