I don't know if this is a common side effect of Mewing. But I find that when my tongue feels fully engaged against my palette from front to back, my smile widens a little because the corners of my mouth, which normally angle down slightly, are stimulated into holding themselves up naturally. I don't know why this is but it's quite a pleasant effect!
Yes this is good. I think the muscles of your tongue have a way of lifting up everything on the face.
If you look at some people, as they age, their chins get sort of bigger over time. I think poor tongue posture is partly to blame, because when they smile, they are using their chin muscles a lot. Instead of their tongue. Look at some of the celebrities starting in their late forties.