Why do you think that every time Charlotte is mentioned, Mrs Bennet disparages her?

"First of all he asked Miss Lucas. ... but, however, he did not admire her at all: indeed, nobody can, you know." (ch 3)

"Did Charlotte dine with you?"

"No, she would go home. I fancy she was wanted about the mince-pies. For my part, Mr. Bingley, I always keep servants that can do their own work; my daughters are brought up differently." (ch 9)

"It is a pity they are not handsome! Not that I think Charlotte so very plain -- but then she is our particular friend." (ch 9)

"...but you must own she is very plain." (ch 9)