She wasn’t schizophrenic because what she was enabled to do, no schizophrenic would have been able to do.
She didn’t have mental issues either, because she did not manifest in her actions things that are normally associated with mental illness. (Besides the claim to have spoken with a few saints and with God- but if that alone denotes mental illness than so were Moses and all the other prophets and saints. So that alone never denotes mental illness -unless you are a staunch atheist. I guess to them this would all be crazy.) She was said to be bright and alert and controlled in temperament. She was not emotionally or behaviorally unstable. Most of all she was virtuous.
A lot of the movies these days portray her more as a some sort of wild male-like modern day feminist crazed warrior, but be warned that most of these movies were made only for the sensationalism of an idea and not made to show the truth about her. She was definitely NOT a feminist, nor had any of that feminist spirit you see today. She in fact dissuaded women who tried to do what she was doing because she knew it was not a woman’s place and that she was only occupying that role because the Lord had specifically instructed her to do so.
St. Joan of Arc is a fascinating saint and I highly recommend you read a Catholic biography on her as you will see quickly by all that was said of her and noted of her she could not have been mentally ill but most definitely a saint of God, her heart united to His.