Unlike Little League, where the mercy rule is often applied, there isn't a standardized MLB mercy rule. In professional baseball games, the sport's competitive nature is prioritized, and games are typically played to the end, no matter the score difference. Even if a game gets out of hand, the caliber of players in the major leagues is too high to employ a mercy rule.
Regarding the question, "Is there a mercy rule in MLB?" the answer is typically a clear "No." In Major League Baseball, the nature of the game allows for unexpected turns. Even if a pitcher has a bad day and gives up many runs, the length of MLB games offers plenty of chances for the opposing team to catch up.
The manager of the team often uses these situations to test a player's resilience or to save their bullpen. This level of unpredictability and depth in the MLB makes the use of a mercy rule unnecessary.
The Little League Mercy Rule spares kids from the embarrassment of a blowout loss. In MLB, multi-millionaire athletes don't get their feelings hurt easily. They earn massive salaries, and sometimes, their awful play should embarrass them. There are too many good players at this level for blowouts to happen consistently to a team.