Katrina and the FEMA response is not why the American people consider the B*sh administration to be a failure.
The biggest reason B*sh is considered a failure is because he sold the War in Iraq to the American people by claiming that Saddam had plans to attack the United States, when his genuine motivation was to enjoy an easy military victory and establish a model democracy in the region.
When it turned out that the reasons were proven false and the military victory was not as easy as B*sh expected and they could not make Iraq a model democracy, then the American people rightfully concluded that B*sh-Cheney-Rumsfeld were stupid, dishonest and incompetent.
Katrina was an event that gave some former B*sh supporters a reason to finally abandon the failed cause, but it was inevitable that the vast majority of Americans would conclude that B*sh was a likeable incompetent who trusted Cheney and Rumsfeld too much and the results speak for themselves.
Where Ignorance is Bliss, 'Tis Folly to be Wise.