I did not find a meaning of "evening star" like you, but several English name websites claim that this is the name of the Mayan moon goddess.
I tried to verify this. Here is what I found:
The name of the moon goddess of the Mayas was Ix Chel. Information about her can be found in a large number of places, e.g. here:
http://en.wikipedia.org/wiki/Ix_Chelhttp://www.athenapub.com/ixchel.htm
The purported meaning of her name as "she of the rainbow" can be verified with this little dictionary of Mayan words:
http://www.mesoweb.com/resources/vocabulary/Vocabulary.pdf
Because "Ix Chel' is pretty close to "Itzel" and because of the many sites that mention the goddess (independently from the "baby name" websites with their reliability problems) I would say, yes, Itzel seems to be of Mayan origin.
Rene www.AboutNames.ch