The 'ugly American' moniker has been around for about as long as America has been a country; a land of immigrants, who didn't have any kind of caste system or, social class constraints. Those in other countries who came across an American was either amused by their hutzpah or, put-off by their 'lack of class and place'. Post WWII, as the rest of Europe was rebuilding and dealing with economic challenges of re-assembling their societies, the US's economy post-war improved and the overall standard of living skyrocketed. US serviceman, diplomats and businessman were posted all around the world, their higher standard of living combined with a chip on the shoulder of 'winning the war' attitude rubbed a lot of folks wrong. Understandably resentment built when boorish and obnoxious American twenty somethings (we're all jerks at one time in our twenties), overseas for the first time, interacted with the locals.
Fast forward today and in my 40+ years of travel, I've seen three types of ugly Americans. The first is the teenager/college-ager, away internationally for the first time. Not terribly self-aware, full of confidence and constantly looking for a good time, their ability to 'know the temperature of the room' and act appropriately is pretty low. Flip-flops, headphones, loud talking, wearing sweats & tees and creating a scene...typical youth. Another group are senior travelers, for some, their first time international traveling also; every conversation begins with 'Back in the States...', or, the endless string of complains, whining and criticisms. Usually traveling in packaged tours, mass groups moving like an amoeba, not really interacting with the country they're visiting. Lastly, the final group is the Know-It-All American, this person isn't age specific, they're usually well-traveled however they'll tell you about all their travels, what you're doing wrong, and their interaction with locals comes across as forced, disingenuous, being cute and cheeky with the locals but, they're really are just making an donkey of themselves. Usually unassuming, however crack the shell and it's a big reveal that they're much more aware or, 'woke' than you. You just don't know the world like they do and 'they know how things really are', they've got the answers to the world's problems. Social media has helped develop this creature.
Fear not, Americans come from a large country with a very big population, which means generalizations are just that, too general. There's other big countries with massive populations that travel too and...we're not too bad. Social media, internet and easy to watch videos have made travel much more accessible which helps round-off those edges and corners which people may have had before their journeys. American's are traveling to more obscure places (like the Germans) and willing to spend more time in places (like Aus & NZ) than past generations. Lets acknowledge that we've become better travelers, get off our moralistic high-horse and help those who may not be so keen or, aware. Nobody likes a scold or, a high-minded, look-down their nose type.