It means the media need to quit nit-picking and pay attention to the important issues of the campaign. Why do they bother with stuff like this? It's not as if people are not gonna vote for him because he didn't pick the best word. Seriously, if I had a car (thank you economy) I'd put a "Nobama" sticker on the back of it, but the dude had a clumsy-tongue moment. It's getting a bit ridiculous! To really answer your question, it means it wasn't the best outcome.
Optimal means the most favorable, so "not optimal" means the outcome was not the most favorable. What Obama was trying to say was that the death of the 4 Americans wasn't a good outcome, but according to some people, the choice of words wasn't on point.
No one that loves America and defends its citizens would refer to those killed in Benghazi with such a "soft" word as "optimal." Obama's colors have shown through time and time again. We will show him at the polls in Nov who we want to represent America and her values.