exaggerated facial expressions are a symptom of strokes, just saying, the list of symptoms is getting pretty long.
Its also definitely a character. The negative traits of Phil are definitely his real personality shining through, the rage, the pettiness, ignorance, greed, etc. But all of the excitement, goofiness, feigned surprise, its just him acting how he thinks successful streamers should act.
That one time he left the camera on after the stream you could see the real Phil, just miserable, with a look of confused contempt with his arm slung over his head like some hateful ape.
You can see what he was like when he was younger in old SF tournament vids other people took, sometimes sorta normal or just reasonable levels of salt, but he's become some sort of Id-driven shadow monster with no self-awareness.