I found this passage highly interesting. On the one hand, she has all the praise of them from Mr Knightley, which she passes off very quickly - 'very creditably she believed - she knew Mr Knightley thought highly of them'.

And on the other hand, her own imaginations:

must be doing her harm
they must be coarse and unpolished

The use of the word 'must' in both of these sentences proves that she is convincing herself of these thoughts, and it is clear that there is no real foundations for them. From other posts below, it rather seems as though the Martins have been doing Harriet nothing but good! Every ungenerous thought Emma has of Mr Martin has no basis at all in fact, and we see this in Chapter 7.