There's nothing wrong with that per se. However, there's a difference between being a lovable curmudgeon, like McCoy, and being downright cruel. And given Data's innocent nature, she was definitely cruel, needlessly so, to him.
But look at the context. She didn't believe Data was alive, sentient, or anything else even close. When she first encountered him, she thought that he was nothing more than a computer that happened to take the general appearance of a human being.
Would you think twice about saying something cruel to the computer you are using to visit TrekBBS? Would you be worried about hurting its feelings? I doubt it, because you know that, despite the things it can do, it is really an inanimate object. She viewed Data in much the same way.
Now, the idea was that her perception of Data was wrong, and as the season progressed she rather quickly began to realize that. Her behavior toward him changes markedly in just a matter of a few episodes. But in the beginning, her behavior is perfectly understandable. Not because she's a cruel person. Because she doesn't realize what Data is.
And, frankly, I find that to be reasonable. Would there not be more than a few people who would probably take some convincing that Data is, in fact, a sentient life form?