that Mr. Collins had no more intelligence of his cousins' being beautiful than of their being amiable as he describes them in his letter. I think he assumes or is determined to convince himself that they are both, & that their being either is convenient but not essential. My guess is that the chief of their appeals is availability, social status ("gentlewomen")& lack of funds to attract other suitors.
It may be that he had encountered an acquaintance who described them at some point ( even the person driving the carriage the last few miles to Longbourn) but I believe that Mr. Collins' servility would easily trump his honesty. He would never let lack of information stand in the way of "those little delicate compliments which are always favored by the ladies"