I don't think Valak's regular form IS of a nun...
...but I figured that Lorraine Warren touched the possessed Frenchie and saw Valak's vision of Ed's death, Valak took that form at that moment to MOCK Lorraine's faith. But I fail to see why Valak would be taking the form of a nun before then... unless it had just been hanging around with nuns for so long, so it got used to the habit.