Lizzy knew her mother would embarrass her, no matter how she took it. From ch.59, when Lizzy and Darcy are deciding how to break the awful news to her family:

She could not determine how her mother would take it; sometimes doubting whether all his wealth and grandeur would be enough to overcome her abhorrence of the man. But whether she were violently set against the match, or violently delighted with it, it was certain that her manner would be equally ill adapted to do credit to her sense; and she could no more bear that Mr. Darcy should hear the first raptures of her joy, than the first vehemence of her disapprobation.