On my first readings of P&P, many years ago, I assumed that Lizzy meant she was never surprised by compliments to Jane, because who wouldn't love her sister! But in later years, I tend to think she is referring to compliments directed to herself. Lizzy is a "studier of character." She is not surprised by compliments not because she doesn't get any, or because she is explicitly waiting for them, but because she (feels she) is perceptive enough to recognize when someone likes her, even before he says anything!

So as Nan said there is irony in her failure to recognize Darcy's interest. But there is further irony that the quality she is perhaps most proud of (discernment) has failed her.