I think your opinion of the polling system will be driven by your own perception. If you love Trump, you will be psyched by those approval rating percentages, thus admitting they are a good measurement. But a week later if the same company puts out polling numbers that say something you disagree with you suddenly doubt their validity.
It is driven by our desire to be correct, but also to be part of the consensus. It is comforting to be a part of the majority.
There is also mystery around something we don't comprehend that makes polling even more hazy. If you don't understand the math / formulas / etc. behind the process it can appear easily manipulated. No one would doubt that Kim Jong Un, Xi Jinping, or Vladimir Putin purposefully manipulate polling to say what they want it to say. So, dependent on your trust of the source you can start to believe in a broader conspiracy to mislead the public. If anything Trump (cleverly or accidentally) has convinced his base to completely distrust any and all media. So, because an American president can't manipulate the poll numbers he has instead successfully convinced A LOT of people the polls are inaccurate or manipulated by his enemies.