JA is talking about is;

"..want of talent and taste which confined their employments, unconnected with such as society produced,"

Wouldn't this mean that the Middletons lacked taste which was 'conventional'?

I like the couple in their different ways. Even Lady Middleton, who had nothing to say except about her children, is well liked by me, because of in what is written about her in CH 6;

"They were of course very anxious to see a person on whom so much of their comfort at Barton must depend;"

This alone makes her a woman of having a natural taste for not being mean like Fanny Dashwood on whom the future of the Dashwood women depended, and who showed a very mean taste in her dealings with them.
Lady Middleton has 'let' her husband rent the cottage at reasonable rent, send baskets of produce and gifts of game, that too quite often we are told.

So while Sir John showed good taste in his behaviour toward them so does his wife.

In fact I am going to remember these lines about her for the rest of the reading whenever I have to make a comment about her.