"Playfully cruel" is a bit harsh, but I'd have to go with that one.  Juliet's nurse is just joking with Juliet.  They have such a tender relationship, and it comes through with such vivacity in this scene. 

"I am aweary, give me leave awhile. / Fie, how my bones ache!  What a jaunce have I had!" (2.5.27-28)

The nurse, of course, is not one to whine and complain; therefore, the reader can assume that the nurse is doing this on purpose to joke with Juliet.  This same kind of fake whining and complaining goes on and on:

"Lord, how my head aches!  What a head have I! / It beats as it would fall in twenty pieces!" (2.5.49-50)

You see, there is a bit of dramatic irony here.  Both the reader and the nurse know that the news is good.  Juliet, of course, doesn't know this.  One can't blame the nurse for having a bit of tender fun.  However, my very favorite part of this scene is a bit later:

Juliet. I' faith, I am sorry that you are not well. / Sweet, sweet, sweet nurse, tell me, what says my love?

Nurse. Your love says, like an honest gentleman, and a courteous, and a kind, and a handsome, and, I warrant, a virtuous--Where is your mother?

Juliet.  Where is my mother?  Why, she is within. / Where should she be?  How oddly thou repliest! / "Your love says, like an honest gentleman, 'Where is your mother?'" (2.5.54-62)

In fact, if you want to see this scene acted to perfection (in my opinion) check out the Claire Danes / Leonardo DiCaprio version.  The tenderness and the laughs come so easily in this scene!