I have run into really good people in poor countries and a few rip-off jerks. But, I have run into the same variety here in the U.S. I am more careful in 3rd world countries (or whatever the politically correct phrase is now) because any place where there is a lot of poverty, there are more people desperate to get their hands on your money, one way or another. And, I would never depend on the police in these countries because they are sometimes corrupt beyond imagination. They make our CEO's, politicians, etc. look innocent.
Cozumel has gotten a tad crowded and more of a tourist trap than it used to be, but it still has a lot to offer. Please give it another chance some day.