They're laughing because they didn't expect such an honest answer, they all (or most) agree but just didn't expect it, and as Secondeuce says he tops it off with being quite sheepish saying it.
EDIT: to add: They like him because of this answer, and are laughing with him, not at him.
What? Look at the video again put mouse over 17secs to 19, there is exactly 2 secs of silence from everyone..
federer laughs at his arrogant statement for about 1.2secs..and then all reporters join in.. very, very awkward indeed.