> He only won thanks to a) a quirk of the American electoral system and b) his opponent also being historically unpopular, by nature of having been in the public eye on one side of the US political system for 25 years.

I can certainly agree with b) but a) is not a very useful way of looking at things. if you went back and changed nothing else about the election except making popular vote the win condition, hillary clinton would obviously have won. but this small modification would have totally changed the campaign strategy of every candidate. people who didn't bother voting might have voted. donald trump might have won anyway with a different strategy. we might not even have been choosing between trump and clinton in the first place!

not saying the electoral college is good, just that this isn't a strong argument to the contrary.

I agree that it isn't a useful argument, but it makes it difficult to argue that Trump had a majority of Americans thinking he was competent to run the country.

> He only won thanks to a) a quirk of the American electoral system

What quirk is that? It has a lot of quirks, and I think you mean one in particular but not sure which one.