It would be great to have Mulan show affection for someone, but she only just lost the man she loved to a wraith, and her commanding officer to an evil witch, so maybe he coping mechanism is to emotionally close off altogether (this at least explains why she seems so cold all the time). The only time we see her before Philip and Lancelot’s deaths is when Philip is risking life and limb for another woman, which probably made her at least a little bitter.
I don’t believe that Mulan is a villain – betrayal is definitely not her style. Deception, maybe, what with joining the army as a guy and everything, but betraying the remnants of her land to Cora is not something she would choose to do, no matter what Cora offered her. She’s far too much about duty and honour and sacrifice to betray good people.
If she is a villain, it would be because Cora is forcibly controlling her, but even that could be a stretch – if Cora has her heart, it doesn’t mean she can’t rebel. Graham rebelled against Regina in order to free Charming and reunite him with Snow.
If Mulan is working with Cora, it’s not of her own free will.