I know we can use "how heavy" to ask something's weight. But I am wondering can we use "how heavy" to ask for a person's weight? Is it appropriate?
You could use this question, but it's a bit risky: it could be seen as a biased question. These are the definitions of "heavy" from the Merriam-Webster dictionary:
having great weight : difficult to lift or move
large in size and weight
having a particular weight
The first has clear overtones of "overweight", the second of "large"- so there are two inappropriate meanings that are more widely used than the meaning that you intended.
As Jim says, "How much do you weigh?" or "What is your weight?" are much safer, and are much more widely used and understood.