Regardless of which candidate in the upcoming presidential election any hat worn by Bosa (or anyone else) would have worn before, during, or after the game, it’s a clear violation of the rulebook. As flagged by Alex Simon of SFGate.com, the plain language of Rule 5, Article 4, Section 8 applies to post-game messages from players:

The 49ers defensive end crashed a postgame interview while wearing a "MAGA" hat. In the process, he broke an NFL rule.