In my opinion Emma's assessment of Harriet is fairly accurate because Harriet's actions and conversations show this. Emma has observed that: she "was not clever, but she had a sweet, docile, (as in easily led) grateful disposition; was totally free from conceit;...though strength of understanding must not be expected." I, personally, as a reader, see these things in Harriet myself.

During their conversation about the Martins, to my way of thinking, as far as Harriet is concerned, she was visiting her friends, the Martin sisters, who have a brother who is nice to her and whom she is inclined to think well of... It is obvious to the reader and Emma that Mr. Martin is in effect courting Harriet: she "was very ready to speak of the share he had had in their moonlight walks and merry evening games; and dwelt a good deal upon his being so very good-humoured and obliging. "He had gone three miles round one day, in order to bring her some walnuts, because she had said how fond she was of them -- and in every thing else he was so very obliging! He had his shepherd's son into the parlour one night on purpose to sing to her." I do believe that Harriet is responding positively to his actions but I don't think that she has come to any self-realization of a possible romantic relationship with him. She has yet to understand what her feelings could possibly mean.

As Emma purposefully discusses Mr. Martin someday marrying some, as-yet unknown person, there is no indication in Harriet's reaction or speech that Harriet would have liked to think of herself as being that person or that she was bothered that it could have been someone else. Emma herself saw no reason to think that Harriet had any designs toward Mr. Martin: "Emma watched her through the fluctuations of this speech, and saw no alarming symptoms of love. The young man had been the first admirer, but she trusted there was no other hold, and that there would be no serious difficulty on Harriet's side to oppose any friendly arrangement of her own." As we can see by this last sentence, Emma has already decided to match Harriet up with Mr. Elton in accordance with her announcement to find someone for him.

When they meet Mr. Martin on the road, Harriet seems to have no problem with the fact that Mr. Martin has not yet gotten the book: after talking with him for a few minutes, "Harriet then came running to her with a smiling face, and in a flutter of spirits, which Miss Woodhouse hoped very soon to compose." To me that indicates Harriet to have been quite accepting of his having forgotten the book and was fine with the fact that he planned to get it the next day.

Emma sees that Harriet is vulnerable to Mr. Martin's designs on her and this does not fit in with her plans so Emma then leads Harriet through a skillfully crafted conversation designed to overpower Harriet's resistance to think poorly of Mr. Martin (as Emma wishes) even if he doesn't come up to the standard of Mr. Knightley. By the time Emma mentions his lapse in not obtaining and reading the book, Emma has done her job well enough for it to have the desired effect on Harriet. And by the end of the conversation, Emma has transformed Harriet's smiling, fluttering spirits for Mr. Martin to blushes and smiles for Mr. Elton.

I see Harriet as pretty innocent at this point though this could change if she is around Emma's influence long enough to pick up some of Emma's manipulative ways.