I'd say that Mary uses Dr Grant as a vivid example of why she does not respect the Clergy, but his defects are not the source of her prejudice, which I believe has been shaped by the attitudes prevalent in the segment of London society she is familiar with.

There is an interesting conversation between Edmund and Mary on this very topic in ch.9 (when they are on the terrace after leaving the chapel). The whole conversation is relevant to this discussion and worth another read, but I'll just quote a bit of it. Mary speaks first:

“You assign greater consequence to the clergyman than one has been used to hear given, or than I can quite comprehend. One does not see much of this influence and importance in society, ...... How can two sermons a week, do all that you speak of? govern the conduct and fashion the manners of a large congregation for the rest of the week? One scarcely sees a clergyman out of his pulpit.”

“You are speaking of London, I am speaking of the nation at large.”

“The metropolis, I imagine, is a pretty fair sample of the rest.”

“Not, I should hope, of the proportion of virtue to vice throughout the kingdom. We do not look in great cities for our best morality."