Is it against the law in los angeles to display written profanity on a shirt or car or anything?

Yes, as long as it's not on private or government property. If you walk into say a McDonald's and they ask you to take it off or leave you probably should anyway. Or say a government building. But just in public like walking around is perfectly legal. You have to remember store policies may not be law, but if someone there (Like a security guard) asks you to leave you do have to leave, other wise it's tresping. But you could file a complaint for discrimination.