It seems that the citizens of Mayberry were targeted by swindlers of one sort or another quite frequently. Could this reflect the truth that small town people tend to be especially vulnerable to con artists?

Not being from a small town, I couldn't say. But back 50 years ago, I think most people were more willing to trust others, and take them at their word, and that could lead to being cheated by con artists. And while we like to think things have changed, and I'm sure they have, the Internet has given rise to new ways of being cheated.

Small town folks are trusting and can seal a deal with a handshake.  When outsiders come in they sometimes forget that big city folk aren't as worthy of their trust.

And yes swindlers do prey on this.