Why would she call Bubbles a lady? Isn't lady a title for nobles?

Native speaker’s answer
Rebecca
That's right, "Lady" is usually used for nobility. Here it's used for humor for the audience and perhaps as a way to show differences in manner. The unicorns seem a lot more successful and esteemed as if coming from a high-end family, so using "Lady" suits their characters. Perhaps a more appropriate title for Bubbles would have been "Miss," It's formal but not as formal as "Lady." Ex: Miss Bubbles will be joining us for dinner tonight. Ex: I want to be a lady in a mansion one day. I just have to marry a prince.