I realize that Mr K is not a social reformer, but he is not as hidebound as Emma.

Mr K mixes with people "below him" socially because of the natur of his owrk as local squire. So he is bound to be more tolernat of a lack of social graces than soemoen like Emma - and he has less interest in "pretty manners" anyway. Emma isn't in any case decrying Rob Martin's goodness or intelligence or qualities as a farmer, but simply saying tath he's not as elegant as "real gentlemen" and as such, she thinks he's not quite right for her protegee..

But all teh saeme Mr K. does not approve of Emma's taking up Harriet, partly because he feels it is no kindness to the girl, but also becauase IIRC he says "she was placed in Mrs G's care, and expected to mix with Mrs G's acquaintances" and it is really best to leave her tehre...Which sounds to me like "She's obviously of a lower rank in society, that was what her family or father expected for her, that she woudl be among the "second class or third class" of Highbury society, and really you should leave her there.... Its not really "on" for people to move up in society all that much....