Maze is mentally strong for sure, because getting back from injuries is not for the weak hearted. Appreciate him for that and the amazing comeback against Chen Qi.
However, when people say Samsonov is not mentally strong because he loses in the decider, that's just plain wrong. Cut him some slack. He has been in Top-10 for 15 years.
For me, not showing emotions means the player is unemotional, which leaves him in a better position to assess what went wrong, and what to do and what not to do. Just because somebody's quiet doesn't mean he is great. But 24 pro-tour titles? Gotta be kidding. Now, I don't want to change this is in to a statistical debate. Let me put it differently.
Being unemotional and having the ability to detach oneself from the heat of the moment is what separates the good from the great. You would rarely see Wang Liqin/Ma Long kicking the match table. This is common to all sport except for maybe boxing/rugby. Concentration is the key to table tennis and being emotional does not allow you to focus too well. It is better to feel angry, keep quiet on the exterior and channel the anger to focus harder. Sorta like taking the anger out on yourself than objects and/or people around you. At least that way, your opponent keeps guessing what's in your head.