I've done a little research but couldn't find the reason of why was she called "maid".
Wasn't she considered a heroine by the French (altough I believe the Frenchman referred to her as "la pucelle of Orléans, like "the virgin of Orléans") ? Was "maid" a derogatory word at the time, hence used by the English to belittle her?
EDIT: Thanks for all the answers and sorry for the bad question.
I think it's just a translation of "la pucelle of Orléans" - it's just another way of saying virgin. The online etymology dictionary does say that maid in the sense of domestic help originated around 1300, but it makes the connection between pucelle and maid when referencing Jeanne d'Arc.
Maid means young woman/virgin. It wasn't derogatory at all. It's like saying "the young woman of Orleans".