He started with (I think) the lowest approval numbers in modern records so to an extent it was probably priced in from the outset.
https://www.realclearpolling.com/polls/approval/donald-trump...
For one, if they dip as low as they might given the expected economic hardship and the other cultural issues, you might start seeing rioting.
For another, I don't think it's a given this is Trump's final term. There already is a congressman trying to get in legislation to change the constitution to allow a third term. Even if that fails, the SCOTUS has already taken some egregious decisions in relation to Trump (full presidential immunity even for political assassinations) - what they might think they can get away with in the next four years is anyone's guess.
Amazingly, so far, it hasn't hurt his approval numbers.