Queen (n)
1) the wife of a king
2) a woman who rules over a monarchy in her own right; female sovereign
3) a woman foremost or judged to be foremost among others in certain attributes or accomplishments, as beauty, etc.
4) a place or thing regarded as the best or most beautiful of its kind

Obviously, Queen Zeal is the queen of Zeal, so that's why her title is queen. But, a queen is also someone regarded as the best or most beautiful. I thought this was very interesting because based on her character design, Queen Zeal is gorgeous. She also views herself as the best and most powerful person in the world.

Zeal (n)
1) intense enthusiasm, as in working for a cause; ardent endeavor or devotion; ardor; fervor

This is where her name gets really interesting. Queen Zeal has intense enthusiasm about taking over the world and using Lavos. She is the queen of this, or rather, the ruler of intense enthusiasm. Pretty neat, huh?