Anthony Hopkins mocking Jodie Foster on set is a form of bullying because it is disrespectful and belittling. It is necessary to have a comfortable environment to be able to work well, but bullying in the form of mocking can disrupt the environment and create a toxic environment. Although mocking may be seen as a mild form of bullying, it is important to address it as a type of bullying to stop the behavior. This will help maintain a respectful environment among all co-workers.
No, this would not be construed as bullying. Most top-tier actors purposely put themselves into stressful situations to enhance the mood needed for the specific scene or for what the particular is supposed to be feeling. I think in this situation this was what was going on. It was certainly nothing nefarious.
Unless Hopkins's intent was to intimidate her, then it doesn't constitute bullying. If he was merely establishing a rapport as the two characters they played, his behavior doesn't qualify as bullying. However, if it was upsetting to her she was certainly free to confront him and communicate her needs to him.
Acting is a profession in which you have to constantly be improvising. Hopkins (or rather, Hopkins playing Lecter) wanted to get a genuine reaction out of his co-star in a natural way. Because Foster didn't know it was coming, it works. That's the beauty of improvisation. Both of those stars were being paid very well to ACT in a major Hollywood movie. If Hopkins had continued to follow Foster around after the cameras were off, THEN it would be bullying.