Emma thinks the Cox girls are vulgar, from dictionary.com—characterized by ignorance of or lack of good breeding or taste. I believe that Emma is criticizing their taste and so anyone who wished to marry Mr. Martin has bad taste. Emma has owned this sentiment about Mr. Martin from Chapter 4:

"I think, Harriet, since your acquaintance with us, you have been repeatedly in the company of some, such very real gentlemen, that you must yourself be struck with the difference in Mr. Martin. At Hartfield you have had very good specimens of well educated, well bred men. I should be surprized if, after seeing them, you could be in company with Mr. Martin again without perceiving him to be a very inferior creature -- and rather wondering at yourself for having ever thought him at all agreeable before. Do not you begin to feel that now? Were not you struck? I am sure you must have been struck by his awkward look and abrupt manner -- and the uncouthness of a voice, which I heard to be wholly unmodulated as I stood here." (Chapter 4)