Mr. Rochester actually has numerous pet names for Jane; he calls her "malicious elf," "child," "sprite," "provoking puppet," "witch," "skylark," and various other names depending on the mood he's in. He sometimes calls her "Janet" as well, which is basically the diminutive form of Jane; in fact, the name "Janet" literally means "Little Jane" or "Little Joan." This is similar to how someone named Elizabeth might be referred to as Lizzie by their family, loved ones and close friends. Thus, it is likely that "Janet" is essentially Mr. Rochester's pet name for Jane, a way of expressing his affection for her.
Some readers think that Mr. Rochester's nicknames for Jane are not as random as they might seem, as Brontë often hides meaning even in the smallest of details. It is sometimes suggested that Mr. Rochester calls Jane "Janet" in an attempt to alienate Jane from her "plain" persona. It's a fact that Jane enjoys simplicity; however, Jane is only plain in her style and appearance. Her spirit, like Mr. Rochester's, is passionate, full of "soul" and "heart." Some readers believe that Rochester calls her "Janet" to try and transform her and make her conform to his own image of her. This desire to change, and perhaps control, Jane is echoed in his desire to shower his bride-to-be in fine clothes and jewelry—a desire that make Jane profoundly uncomfortable.
Others will disagree, however, and argue that though he gets a little carried away at times, Mr. Rochester loves Jane just the way she is. From this perspective, his various little names for her (including Janet) simply further showcase the love and fondness he feels for her. Ultimately, these nicknames say more about Mr. Rochester's own personality and quirks than they do about Jane.