Mendoza has a genetic disorder. His face is always flexed in certain spots - looks like a "smirk", and it makes him look odd. I had a friend in junior high who also had the similar genetic facial expression as Mendoza. My friend looked like he was always smiling. Sometimes teachers would yell at him to be "serious" and stop "smiling" at serious things. My friend would then have to talk to the teacher after class and explain to the teacher about his genetic problem.
I can't remember the name of the disorder.
Mendoza might get seriously injured in the future, in the NFL, but through the pain he will likely still have that odd, happy smirk.