There are of course ironies in the discussion of whether America is great or not. Michelle Obama was proud of being an American for the first time in 2008, and now in 2016 insists that America is great, not needing to be made great. What changed, other than her personal fortunes. It reminds me of the admonition repeated every four years that this is the most important election in our lifetime. I think it feels that way to the candidates, because it likely is the most import event, never mind election, in their lifetime. Not necessarily in mine, though. Next irony: It has usually been conservatives who insist that America is great, while liberals talk about America living up to her promise. Trump isn't a 180-degree reversal of that, but it's a go 120 degrees, anyway.
It’s posturing and positioning, of course. Most Americans think their country is great-but-not-great, meaning different things by the same words at different times. That’s the beauty of such things politically, because people can feel sure that Trump means the same thing that they do about what used to be great about America and will be in the future, while others believe that they agree with Michelle about what is already great that doesn’t need fixing. If you let people talk long enough, they usually let you know what they really mean, but most voters have long since left the auditorium by then. They know already, they are sure of it.
Also, everyone thinks they know what the other guys really mean when they say it. That’s not always unfair, if you have some actual data to work from, not just your fantasy of how stupid and evil they are. Michelle Obama likely means ten things when she says America is great, but the one that she always mentions is that there are finally black people in the White House. Trump is the opposite, so far as I can tell, alluding to many things but keeping it vague.
Side note: Best example in my lifetime of the media having cried wolf for years about Republicans, now shocked that a lot of Americans are ignoring them.