I think it's clear that the approval ratings Trump gets are more about disapproval of the rest of politics. When you have every politician getting rich somehow while your life gets worse and worse, a lot of people will want all politicians punished. Trump is that punishment, and many people are excited to see the political and professional classes suffer. That is the approval rating.

The irony being that Trump enriches himself and rewards politicians [mainstream or otherwise] for corruption to a greater extent than any previous politician, and doesn't even try to hide it. The people that are happy their savings and/or chances of making rent next week are being eroded to enrich Trump insiders because at least random mid level professionals and Hispanic people with autism awareness tattoos are suffering more deserve everything they get.