Obligatory protest that we don't know for certain Mr. Hat-and-Cloak is Quirrell.

The way I interpreted that, he thought that if he first showed up looking innocent and good, she would have suspected that the outside was a sham. But after she revealed that she really doesn't think that outsides are always deceptive, he changed his modeling.

Though that would still mean that he mismodeled her, yes.

My thoughts on the mismodelling* of Harry are much more speculative.