Again, public opinion is manipulated by the media. Prominent Leftist authors have written books on that subject such as Does the Media Govern?
Polls are used to manipulate public opinion because, and marketers know this, most people don't want to be the odd man out. Most people want to be part of a group. How do you think the fashion industry works? How about peer pressure? US politics is about big money and big power. Consider the billion dollars spent by the Party's and the candidates on the US Presidential election.
Public opinion polls do not validate Obama's job performance as President. People spend lots of money on diamonds, Mercedes cars that are sold cheaply as taxis in Central America, Rolex watches, Blue cheese which is a moldy cheese, and marijuana to name a few, because of marketing in one form or another. Also, because of excessive positive media coverage, public opinion polls indicate that some have "bought" that Obama was a good President.
Obama's overall job performance was weak because of the specific things I've listed. I do give Obama a higher ranking than either of the Bush Presidents because I can think for myself and am not a Partisan hack.