A friend referred to his new iPad as "my new playtoy" last week.
It sounds to me like the sales clerk was a serious reader who was asked for her opinion by another serious reader.
Since B&N is primarily in the book business rather than the gadget business, I don't think that it is inappropriate for the clerk to refer to the NC as a toy. It would have been inappropriate if she had called it "high-priced junk", dismissing its quality. But she wasn't really criticizing the NC. She was just saying that for serious readers the Nook Wifi is a serious device and the Nook Color is not.
I think that that is a fair opinion one might have.