Every time I joke with my girlfriend, she thinks it's true. For example, your clothes are ugly and stupid.

This kind of joke depends on character. Some girls are introverted or serious. She will care about anything you say, because she cares. Isn't there a saying that "women are pleasing to themselves"? Beauty is in the eye of the beholder. She wants to be the best and most beautiful in your eyes. Girls who are not particularly masculine will be very unhappy. Even a more cheerful girl sometimes doesn't want you to say that she has too many shortcomings. Because she wants the best in your heart. Jokes can flirt, but don't go too far.