I mean, this is...
Americans (as an "ethnic group") have a stereotype of being a bit stupid, y'know? Yes, it's a stereotype, and it's not really got a lot of basis in reality, but... They don't get irony, they're uneducated, loud, somewhat obnoxious - these are all basic stereotypes of the "American". Well, Trump being elected is going to do three things for that stereotype:
1) It's going to reinforce it abroad.
Only in America would the electorate choose Trump over Clinton. Along with sympathy for those who didn't vote for him (and are going to be hurt by him), there's going to be a fair amount of "Well, Americans are
stupid, so we didn't expect anything else."
2) It's going to push the American people into realising how stupid some of their countrymen really are, because Trump really is the dumbest of the dumb. I think there may have been the assumption that just holding the office of POTUS would make someone smart, and Trump is going to show that to be the fallacy it is. I think he already has, to be honest.
3) It's going to severely embarrass people who think that America is great because of its leaders. I can't see anyone but the most ignorant of people thinking that Trump complaining about losing the popular vote on his third day makes a good impression to anyone. To the outside world, to their wife/mother/sister, to their children.
Outside of that, Trump is also well on his way to proving his word matters little. He's signed the US out of TPP, yes, but he's evasive on renegotiating NAFTA (is that even possible?), and he loses on the Federal hiring freeze no matter what. Either he doesn't create jobs because its so effective, or he barely makes any difference because his "Except the military" waiver means most hiring will still occur.
I mean, it sucks for the people of the US, no doubt. But for the Dems and the left generally, I don't think he could've made a better start.