This is a complex question to answer. Wang Hao was mentally very strong until he wasn't. He came in second in World Table Tennis Championships several times. He did win in 2009 but he came in second many times.
Ma Lin was also mentally strong for most of his career but he also never won a WTTC gold title.
Zhang Jike through 2013 was mentally strong in all the biggest tournaments. But not so much in regular tournaments. There are many times where you could see him give up and just quit in the middle of a match. There were a few times where, in the middle of the match he just stopped and forfeited the match claiming an injury. Maybe there was an injury or not. But I have not seen too many players who were at that level, who, in the middle of a match, just stopped and walked off.
Ma Long, early in his career was said to not be mentally strong. He lost to Wang Hao in the semifinals in 2009, 2011 and 2013 all of which were years many people thought he was the best player in the world. But in those semifinal matches vs Wang Hao, it looked like his nerves caused him to fall apart. Was it? I don't know. Was it Wang Hao doing Jedi Mind Tricks? I don't know. But after 2014, he seemed mentally stronger than anyone. And maybe overcoming nerves like that in your early career and, with experience ending up with a very strong mental game is something that having a strong mental game is about.
It can be said that Wang Liqin had a strong mental game early in his career and well into his later career. But it is interesting that, even while winning 3 WTTCs, he never got Olympic Gold in singles.
So....I don't really know if there is an answer to your question. Perhaps, when a player is on, and nobody seems to be able to challenge him, the player shows mental strength and when the player is not on his game, the opposite and that most top players phase into and out of that mental state where it is very hard to beat that player.
In the 2014 World Cup Zhang Jike vs Ma Long, the match went to deuce in the 7th game. To me both players showed mental strength even though only one player can win.