I've been to Mexico many times and I agree that that film depicted what the place is generally and actually like. You can find really nice hotels on many coastal areas, but, just drive into any nearby town from the resort areas and you immediately are in what Mexico is actually about (it's not the hotel environment), and that's pretty well shown realistically in that film, you're right.
Most people in the U.S., unless they've been there and gone outside the expected niceties of the finer hotels there, never see Mexico, nor realize just how primitive and rough the place is.
I'd never, however, suggest that most Americans DO go outside the secured nicer hotels because it's not a safe country to be just walking around, obviously tourists, obviously Americans. Very dangerous, actually.
I'm sure there are safer places, but not in general and it's not a good idea to even consider going to Mexico without a lot of planning and precautions (including getting all your shots from your doctor before you leave -- ask your doctor about two months ahead what you'll be needing).
And, don't drink the water! No ice cubes, no "freshly washed" salad or fruit, no local water! You'll get really sick, otherwise.
Now that I think back over this, I really never want to ever go back there again.
I think so to.