Well, the guy has never been very popular in the first place. Remember that he lost the popular vote (46% of voters voted for him, compared to Hillary's 48%). Also, after all those high profile scandals and policy failures, it makes sense that the guy's approval ratings would suffer; if a president has, for example, 50% approval, and he makes a high profile blunder, of course at least some of those people won't be happy.

I'll end by saying that Gallup is an independent organization which has been doing these surveys since the 1930's, for both Republican and Democratic administrations, plus similar ones in lots of countries around the world. They aren't exactly "left-wing propaganda".