This is a good question. Steinbeck does not make it clear exactly why Lennie hides the dead puppy but not attempt to hide the body of Curley's wife. The text says: "He pawed up the hay until it partly covered her." When Lennie picks up the puppy he says
"I'll throw him away. It's bad enough as it is."
It is hard to guess just what he is thinking when he says, "It's bad enough as it is." Apparently he has some dim sense that hiding the girl's body would be worse than leaving it to be found. Hiding the body would make it look more like premeditated murder or murder committed in connection with attempted rape; whereas leaving it exactly where it happened, and in plain sight, might help to corroborate the alibi that it was an accident. At any rate, he seems to make a rational decision to dispose of the puppy but not of the girl, because leaving both bodies together would make him look more like an intentional killer than an accidental killer. He may, in fact, be the only person in the world who knows he kills things intentionally and not by accident. As dumb as he is, Lennie has some intelligence, and he may have some dark secrets. We see that he lies to George regularly and manipulates George's feelings of responsibility, guilt, sympathy, and friendship.
There seems to be a question of whether Lennie kills little animals by accident or because he gets some kind of pleasure or satisfaction from doing so. There is even a question of whether he really killed Curley's wife by accident or whether he became enraged because she wouldn't obey him and stop struggling and trying to scream. The following description of the struggle should be noted:
Then Lennie grew angry. "Now don't," he said. "I don't want you to yell. You gonna get me in trouble jus' like George says you will. Now don't you do that." And she continued to struggle, and her eyes were wild with terror. He shook her then, and he was angry with her. "Don't you go yellin'," he said, and he shook her; and her body flopped like a fish. And then she was still, for Lennie had broken her neck.
Does Lennie kill the girl by accident, or does he become so angry that he kills her deliberately?
As far as the original question is concerned, I would say that Lennie is smart enough to decide not to hide Curley's wife's body because:
1. He knows she would be found sooner or later anyway.
2. He might be able to claim he killed her by accident if he does not hide the body, but it will obviously look like intentional homicide if he adds to his crime and guilt by hiding the body.
3. Also, he is getting panicked. He starts thinking about what George told him to do if he got into trouble. He knows he has gotten into serious trouble, and now his mind is fixated on doing what George told him to do.
Regarding John Steinbeck's Of Mice and Men, since answering the question "Why does Lennie throw away the puppy he accidentally killed, but makes no attempt to hide Curley's wife?" it has occurred to me that Steinbeck must have given considerable thought to the aftermath of the tragic event. Steinbeck's description of the scene includes at least two significant details bearing on the question.
It was very quiet in the barn, and the quiet of the afternoon was on the ranch. Even the clang of the pitched shoes, even the voices of the men in the game seemed to grow more quiet. . . . Around the last stall came a shepherd bitch, lean and long, with heavy, hanging dugs. Halfway to the packing box where the puppies were she caught the dead scent of Curley's wife, and the hair rose along her spine. She whimpered and cringed to the packing box, and jumped in among the puppies.
If Lennie wanted to hide the body, his best bet would be to take it away from the barn. Everyone knows he hangs out in the barn to be near his puppy. If the body were found there, Lennie would immediately be suspected by everyone. But if it were found at a considerable distance away, there would be some hope--at least according to Lennie's thinking--that he would not be associated with it. He might bury the dead girl out in the open fields somewhere, and she might never be found. However, "he crept to the barn wall and peered out between the cracks, toward the horseshoe game." He can see that all the men are pitching horseshoes right outside the barn. He would have no chance of carrying the body away unseen.
Hiding the body inside the barn is impossible. The dog has "already caught the dead scent of Curley's wife" even though the girl has only been dead for minutes. If Curley's wife were missing for more than a few hours, Curley would be sure to be searching for her all over the ranch. There is a good chance that the dog would lead him to his wife's body, regardless of where Lennie might contrive to hide her inside the barn. But even if she were not found for a day or two, the death smell would surely attract attention and lead someone like Crooks or Candy straight to her body. When Lennie sees the dog's behavior he realizes that he might hide the body inside the barn but he can't hide the inevitable smell.
Lennie must have thought about hiding the girl's body as well as the puppy and concluded that it would be better for him to leave her where she lay. He had killed her accidentally, and hiding the body--if he couldn't hide it in some distant place where it wouldn't be connected with him--would only make it look more like a murder. This is why he picks up the puppy and says to himself:
"I'll throw him away. . . It's bad enough like it is."
Lennie is obviously capable of some reasoning, even if he has a childish mind and his reasoning is childish.