Chad Jennings (The Athletic) talks to various Red Sox players about the Mookie Snarl.
Jennings writes that "if you watched his debut season with the Red Sox, you might have noticed it before Mookie himself ever did. ... [I]n moments of concentration, determination or occasionally celebration, Betts' upper lip will curl on the left side."
Nobody ever really said anything about it until, I think, 2014 when I got called up. I got some texts from my fam that was watching the game saying I snarled. That's when I became aware of it.