That depends entirely upon the context. There are many situations where not tipping is rude. There are also situations where tipping is rude. “Rude” is a quality related to social expectations. You can be unintentionally rude due to ignorance of the norms, and that doesn’t necessarily make you a bad person. But if you knowingly refuse to tip when tipping is expected for a provided service, then you’re a shitbag.