Actually, tbh, Dravid has a very good point.
Having your shirt tucked in shows proper attitude towards the game and profesionalism. It might be because you guys (SS and Mr. Mxyz) haven't played sports at a high level where the coach emphasizes on the fact that your attitude reflects your performance. By showing you have your shirt not tucked in, it hinders the level of professionalism that is needed at an international level in sports where you represent your country. It also shows one is not keen on following the proper etiquettes of the game.
I have played three sports at a high level, and in each of them I swear to God, my coach has emphasized the need for proper attitude. For example, in Baseball, no matter what it is, practice or gameday, we come to the diamond with are shirts tucked in and are hats on along with all our equipment. You don't have your shirt tucked in, you sit the game.
Having your shirt tucked in may sound like a very little thing to many people, but it is very important and is one of the things needed to showcase proper attitude which in return reflects your performance.