TL;TD
YES, BUT it is worth mentioning that the dog is not always "smiling" when it opens its mouth, they're smiling because they are calm and relaxed, but sometimes it is also an expression of aggressive behavior.
In the challenge, it doesn't really matter if the dog is smiling or not. Your ability to pass the challenge depends entirely on whether the person labeling the dataset thinks the dog is smiling.
How Do I Know If My Dog Is Smiling?
The ASPCA noted, "This is also a gesture where a dog shows his front teeth, but a smiling dog is doing just that. He usually shows a lowered head, wagging tail, flattened ears, a soft body posture and soft, squinty eyes along with those teeth. Teeth don't always mean aggression—it is important to consider the whole body and the context to understand what a dog is saying."