Yes, overweight airplane passenger should be charged more. In fact, if the passenger is large enough, the passenger should have to purchase two tickets. The flight may be full, and then the attendants realize that the overweight passenger is spread across two seats, other passengers are uncomfortably squeezed into that row or kicked off the flight unfairly. Had the overweight passenger purchased two tickets, one for each seat the passenger technically occupies, others would not be kicked off or squeezed into the airplane.
Charges for being overweight seem to be more of a cash grab by the airlines than anything else, I find it hard to believe somebody carrying a little extra weight is going to compromise fuel costs that much. It's more of an excercise in embarrassment than it is one that resolves anything.