So the question is "Does one need to be an athlete to understand the importance of a strong mental game?" Well personally I'd say absolutley not because having a strong mental game doesnt necessarily mean you are an athlete or participate in athletics. You hear it alot that you're going to win the game and overcome obstacles if you have a strong mentality and dont make mistakes and beat yourself. Well that doesnt always have to pertain to athletics.
    There are plenty of activities which require a strong mental game and preparation other than athletics. Say you are participating in a chess match for instance, you obviously need a strong mental game if you are going to outsmart and out strategize your opponent. You need to figure out how your going to react when they make a certain move or how your going to put them in a situation they cannot get out of. That requires some serious thinking and strategizing which ultimatley has to do with having a very strong mental game. Without proper preparation and having the right mindset you may have lost the game even before it has even begun. Another example is playing a game of poker and in order to  win the game or win the jackpot you have to come to the table with a stronger mind than the rest of the players. In order for you to win you have to have your mind focused and set on winning and not letting anything throw you off your game or confuse you because one mistake  can cost you alot of money and maybe get you into some financial trouble.
    So in saying this I think I've proven my point that you do not need to be an athlete or participate in athletics to understand the importance of a strong mental game. There are more things than sports like job interviews, board and card games and plenty of others that require knowing the importance of a strong mental game and knowing how to prepare you're mind and stay focused that do not invole any kind of athletic participation whatsoever.