Rosa's remark about "learning frankness" from Steerforth is, first and foremost, a jab at Steerforth for concealing his affair with Emily. In a broader sense, however, it's also a jab at the hypocrisy of Steerforth (and perhaps the upper classes in general). "Frankness"—that is, naturalness—is almost by definition something innate rather than learned, so the idea that Rosa can become frank by modeling herself on Steerforth would be nonsensical except for the fact that Steerforth's apparent honesty is itself a performance. Although Rosa is not above snobbishness herself, her comments here do undermine the idea that there's anything inherently noble about the upper classes.