In front of the mirror, you can twist and turn to get the most effective pose, and on videos you're moving. In still pictures, however, you're essentially at the mercy of the photographer, who doesn't know how you like to see yourself. Even if you set up the camera yourself and then run to pose, you still can't see at the at moment how you look. In addition, in posed pictures you're liable to be a bit stiff.
Once upon a time there were self-photographing booths in which you sat in front of a mirror, struck the pose and expression you wanted, and pushed a button. I got fairly good results from those, although I'm not very photogenic, either. But like you, I do pretty well in videos.