According to this video transcript, the most important principle for a Dungeon Master like Matt Mercer is ensuring players have fun. While respecting the established rules of the game is generally important, if a rule actively hinders a player's enjoyment or prevents an exciting moment, it's better to bend or bypass it. Being too rigid with rules, especially under pressure from external expectations, can detract from the overall experience, making it essential to prioritize the group's enjoyment over strict adherence to the rulebook.