That's a difficult thing to answer. Certainly I've often wondered if the whole Sabrina Online comic would ever have risen to fame if not for Zigzag's presence in the comic.
The complication is that Zigzag was not created by the comic's Author, Eric W. Schwartz. She was created by Max Blackrabbit. Most often Mary Sue/Gary Stu's are the author's own creation and treated like a pet that they will never bring to harm, or show any significant faults or weaknesses in.
Yes vote: She's generally the life and soul of the party, overpowers everyone else, is wealthy and successful, and seems to have a gift for getting whatever she wants out of people (most of the time). No vote: She's been wrong a few times, one instance of which earned her a black eye from Sabrina.