Can poking fun at a regional accent rise to the level of harassment and discrimination?

Q: “We have an employee with a thick Boston accent. Several of his co-workers imitate him, like ‘Where did you pawk your caw today?’ He doesn’t seem to mind, but I can’t really tell. Since there aren’t any protected characteristics at play—such as race or national origin—would that type of ribbing get us in trouble if we don’t stop it?” – Kevin, Maryland