I think people regard him as non-establishment, because he came from outside the typical political circle. And during the campaign, he refused to listen to his Republican masters, and invent his own platform to begin with. Until it fell out due to his sexism and casual racism scandals.
Unlike Hillary that has earned herself a reputation political player and their pay-to-play scheme (big corporate pay her and her husband for political favour). Trump earned his wealth from property business, inherited from his parent. However, Trump also sided with the Clintons before, as he was a Democrat supporter before sometime in mid 2000s. But, switching party aside, he is still a flamboyant boisterous business man that has been bankrupt twice. But I am still waiting for his "draining the swamp" promise, so the likes of big corporate, military complex industry defense contractors cannot buy influence to instigate war and therefore a battleground to prove their products. His populism is just his TV show antics, which piss off everyone. I was hoping that his offensive statements was for real, to punish us all, the sinners by silencing, but then he can be just trolling instead.
Now, I don't think he is seriously advocating isolationism. He has properties everywhere around the world, including Middle East. He has manufacturing stakes in China and Mexico. I was hoping his trade war is just a bluff, otherwise, the poor people of USA and the rest of the world can't have their new iPhone for just $800. His real estate book (Art of Dealing) is all about "making good deal" - which is him buying properties for a good price, and then renovate it, sell it at the most profitable price. Besides his flaunting personality, his approach to business-dealing is pretty unique. He gets what he wants, sometimes by making purchase schemes instead of the usual "bundling". If his counterpart isn't stupid enough, they could extract a fair deal from it, after all, it is business where you cooperate to make more profit.