While he wasn't always mean, he was definitely strict and often sarcastic, especially towards Harry. He was also the Potions master in the first few films, and his teaching style could be described as intimidating.
However, it's important to remember that Snape's true nature and motivations were far more complex than simply being a "mean teacher." The films reveal a deep-seated history and emotional backstory that explain his actions and motivations, making him a more nuanced character than initially perceived.