We wouldn't want to mess with this guy, that's for sure. That doesn't make him the antagonist, but it definitely doesn't hurt.
Ichabod is the protagonist (even if we're not happy about it), and his goal is to marry Katrina. Brom gets in the way of this by, um, marrying Katrina. Oh, and he also pranks Ichabod and scares him out of town—or so rumor has it. Almost everything Brom does in "Sleepy Hollow" serves to stand in Ichabod's way. And that, Shmoopers, makes him the antagonist.
Okay, so he's not the brightest crayon in the box, but at least he isn't mean. Brom has many more virtues than Ichabod, and he's actually good at some stuff. Ichabod is kind of like a bumbling poser, but Brom has the muscle to back up his bravado. That makes him more likeable than your average antagonist, but it doesn't strip him of the title.