Not necessarily:
Pennsylvania Vehicle Code Section 4524(c) provides in relevant part:
(c) Other obstruction. .No person shall drive any motor vehicle with any object or material hung from the inside rearview mirror . . . as to materially obstruct, obscure or impair the driver's vision through the front windshield or any manner as to constitute a safety hazard.
Title 75, § 4524(c).
That tells me that if it ain't materially obstructing your vision so as to constitute a safety hazard, it ain't against the law.
Hang your teabag!
Sounds awfully subjective.