It's kind of a tough one, really.
I think it sort of depends on how terrible one believes a person will be for the country, I suppose.
I think it ought to be relatively easy to separate the symbol of progress from the actual person, and recognize the value of the symbol...and, as you say..celebrate that the country got together and did it.
But...if you really feel he is a Bad Thing, I think it would be hard to celebrate.
I guess an analogy for me would be the nuance of reaction to Maggie Thatcher's election.
Some feminists took the view that it was a Good Thing, even if she was a horror...others that it was silly to celebrate the election of such a person, even if she was a black, disabled lesbian.
For myself, I could appreciate the symbol while deploring the choice....but could not really celebrate...
It's a bit like, I guess, the thread Lash opened years ago accusing any "liberal" who was not cheering Condi Rice's appointment of being racist. (Though an appointment is less relevant to your point than an election.)
I was pleased to see a black woman in such a position, but was not able to celebrate the appointment, for a lot of reasons, really.
I do not think it racist to be pleased to see a black person elected to such a position, because there is such a genuine history of terrible trauma and discrimination.
I DO, however, think it racist to vote for someone, or celebrate their victory, ONLY because they are white.
That sounds really weird, but I think it has something to do with the real power whites have exerted re black people in the USA (and elsewhere) historically.