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)