It's perfectly fine to be disappointed with it. You were aiming for a certain grade and you didn't obtain it - naturally you will be somewhat disappointed. However portraying this disappointment as if it is the worst thing that happened probably isn't the best thing to do. A lot of people would be very happy with an A and for someone to be disappointed with it, they may feel stupid by the fact they can't even get a grade that someone thinks is bad (which obviously is not true)
So it's fine to be disappointed, in fact it's a good thing because it means you aim very highly, just don't publicly display this disappointment.