Now that she has Robert's fortune to spend, Lucy doesn't have to flatter and fawn over people in the way she always used to. She made her way in the world by 'sucking up' to people who could do something for her. That's no longer necessary with a rich husband. Lucy probably did want to say how she really felt to people, long before now.

She let Elinor have a glimpse behind the mask--the maliciousness etc.--but not everyone had seen that. Even Edward had no idea at all. In Ch. 49, while he and Elinor are talking it all over, we learn that "till her last letter reached him, he had always believed her to be a well-disposed, good-hearted girl, and thoroughly attached to himself."

But Lucy probably kept a lot of her real feelings to herself, out of necessity. I doubt that she enjoyed having the Middletons' children untie her sash, pull her hair about her ears, search through her work-bag, steal her knives and scissors, etc, as described in Ch. 21, nor did she want to sit out of the cards and other socializing in order to make a paper basket for spoilt Anna-Maria--but she had to in order to stay in Lady M's good graces. She probably has a whole bunch of people she'd love to tell off or at least show how superior she now is to them by 'cutting' or socially snubbing and ignoring them.