I find the question too obvious to ignore what it is implying: Did people vote for President Obama the first time because he was black, and why would they vote for him a second time since he is not a good president?
Did some people vote for President Obama because they wanted to see the first African-American President? Probably, though how many is impossible to quantify. However, in light of the fact that all past presidents were from from only one group of citizens, white males, while more qualified women, African-Americans, or other minorities were overlooked, I can sympathize with that choice. In a representative democracy, people get to choose for whom they will vote, and it is only in the last 50 or 60 years that the choices for any office have included candidates who were not white males. Most are very well qualified.
Also consider that if the supposed standard is to vote for candidates only because of their qualifications, then voting for a candidate for any other reason, like simply because they are a Republican or Democrat, is no different than choosing on basis of race or gender.
As to opinions about President Obama's fitness or performance, it is obvious many are based simply on ideology. History will be the judge. Economically, the trend will show that the economy improved slightly and the deficit began to decline. His positions on gay marriage and immigration will be considered as obvious moves in the right direction. Regardless of the quality of the ACA, the US not already having universal health care by 2000 will be puzzling.
President Obama's foreign policy actions will be viewed as fairly consistent with other modern U.S. presidents, with the exception of George W. Bush. His rejection of permanent troops in Afghanistan and Iraq is consistent with every other post-Vietnam administration's approach to regional conflicts. President Obama's use of the military in the Middle East is consistent with the Carter Doctrine adhered to by Presidents Reagan, George H.W. Bush, and Clinton: deployment of force only if U.S. vital interests are threatened by other nations - vital interests being the supply of oil to U.S. and its allies. en.m.wikipedia.org/wiki/Carter_Doctrine
His policy on Israel is not materially different except that he has been more open about disagreements with the Israeli government. The nuclear deal with Iran is considered a no-brainer by the other P5+1 nations. President Obama's approval of the use of drones to kill terrorists in other nations would be considered quite hawkish by some Democrats and Republicans of the past and by some historians in the future.
Many people support President Obama for these reasons, not simply because of his ethnicity or the fact that he is a Democrat.