In context? Because Captain Marvel is an asshole, the kind that isn't motivated by a larger purpose, just a complete dickhead. If she's smiling, you're either asphyxiating, or she's killed you for questioning why she's only arresting black people.
Additionally, she has a pretty poor life, even if most her problems are caused by her.
In reality? Brie is not intimidating when she smiles. At all. And Marvel's a bit too thick to pull off charisma checks required to trick villains, then smack them silly.