I’d be hesitant to call any native speaker’s grammar “bad grammar” in their own language. Grammar rules are constructed to explain how native speakers speak, who learned their native language without learning grammar. The way native speakers speak is designed to be reflected in the grammar of the language and define it; the grammar doesn’t control how a native speaker is supposed to speak. It doesn’t work that way.  TV and films actually just reflect how native speakers really talk with one another, so they are a great learning tool. If you get off a plane in a major US city expecting everyone to sound exactly like what you studied in a language class, you’d likely be sorely disappointed and find yourself unable to understand most of what people are saying around you. That’s why films and TV are so useful. If you can get used to the phrases used in movies, the pace at which the language is spoken, and sound of it, and the idioms, you’ll really prepare yourself for real speaking with native speakers. Humor used in movies and TV is sort of culturally driven, so it may not always seem funny in a translation. That’s where acclimating yourself to the culture comes in. 

I imagine that all languages use slang words or bad grammar in certain situations. Every language has slang or more common speech that is used in informal settings, such as conversations between close friends or family members.  Although proper, correct language is learned in school and used in formal settings, people tend to take shortcuts with their speech, and English is no exception.

Actors must speak the words that are written for them in the script, even if it is not the way they speak in daily life.  Many times the screenwriter is trying to convey a certain social class or economic level of the character, and language is one way to do that.  If a male character says, "Good evening, ladies", I will get the impression that he is a classy, maybe rich guy.  If he says, "Hey, what's up you fine chicks?", I will think that he is lower class and perhaps a ladies' man or a scoundrel.  If you are watching a 'reality show', know that these shows are often highly scripted as well, and the participants instructed to speak in a certain manner to achieve a certain effect.