"How unpredictable you are!" is a perfectly fine sentence. "How are you an unpredictable person!" is not right. Instead, it would be "What an unpredictable person you are!"

How are you an unpredictable person? is a question How unpredictable you are! is a statement, with emphasis on the person being very unpredictable.

What an unpredictable person you are! How unpredictable you are! "How are you an unpredictable person?!" sounds like disbelief.

“How are you an unpredictable person!” would be a question, you would be asking how they are unpredictable. “How unpredictable you are!” is a statement, you would be calling someone unpredictable. Hope that helps!