...Toward the beginning of Ch 7, our JA says,

"The two youngest of the family, Catherine and Lydia,... their minds were more vacant than their sisters',...

They were, after all, teenagers, and demonstrated, minds more interested in the opposite sex, in this case, the regiment - not at all impressed by Bingley's five thousand pounds.

So, when Mr. B says that they are silly and ignorant, I believe much of his thinking is based on their one track mentality.

I'm wondering, upon this rereading,if when he says,
"They have none of them much to recommend them," he's referring to the small amount they'll receive when they marry.