"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!