Remember how high HW's approval numbers were in March '91?
Off the chart!
You are right to point this out. However, in many ways the circumstances are different. First, GHW Bush had a significant “honeymoon” period with the media. So when they did rail against him it had a significant effect. Trump has had everything and the kitchen sink thrown at him from day 1 where day 1 is the day he announced for office.
Secondly, GHW Bush pulled a 180 on “no new taxes” that upset staunch conservatives. There are things Trump has not completed yet, but there is no major promise he made that he has reversed himself on.