Unlike most of the room she knows Harry well enough that even him scaring a Dementor, no matter how surprising, wouldn't make her personally afraid of Harry; she might be worried about what trouble he could cause but she knows perfectly well that he wouldn't do anything to her. Besides it was less of a surprise for her since Dumbledore already told her Harry had developed a new charm.

McGonagall is House Head of Gryffindor.

She is just that unflappable.

Or, you know, relief + dry sense of humour = exactly that kind of reaction as a coping mechanism.

I am reminded of why I prefer British comedy to American - in American comedies everyone tends to be very obvious and melodramatic, while in British the tendency is more towards understated and deadpan. McGonagall's reaction fits perfectly into the latter category, trivialising the entire situation rather than mugging for the audience. (Not that some of the humour in the earlier chapters hasn't been overblown melodrama. Harry's parents leaving the room to have hysterics stands out as the most obvious example)

Or, she's simply ceased to be surprised at the extent of Harry's abilities outpacing her expectations of them.

Or, she’s the head of Gryffindor, and she felt the need to at least appear to put up a brave front in support of her students.

Or, in addition to everyone else's reasons, she's already working hard to maintain a calm demeanor for the sake of Hermione and Harry.

A possibility is that she thinks very fast* and realizes that Harry ought to move away from the dementor (since it is effecting him some) but that it must be done in a way that makes Harry look strong, not weak. Showing that Harry has no problem standing undefended next to a dementor but walked away out of pity reinforces his strength nicely.

*= fast may actually mean that Dumbledore went back and gave her carefully edited information so she could make plans in advance

Or she already knew.