Dogs “smiling” is typically a stress/fear response. Pitties kind of look like that a lot of the time, just because they have tight skin on their face/lips. But, this is more than that as far as I can tell here. You can see the wrinkling and tension at the corners of the mouth. This expression is also called a “fear grimace” or “submissive smile” and can also involve showing teeth, but not always.

Given the context, that it just destroyed a toy (something it may have been punished/scolded for in the past), I would bet it is nervous/scared. It could also just make weird faces - not all dogs are the same.