You don't have any obligation to communicate with your bio mom. She gave that expectation up when she gave you up. You are free to do as much, or as little, as you choose in relating to her.

At the same time, she doesn't have any obligation to be the person you might like her to be. She is who she is. It's natural that you are curious about her. That's why you like to read the letters. If you don't respond to them, my guess is that she will eventually stop sending them. So it's up to you. I'm not sure what your friends expect you to do. You do have a right to protect yourself from emotional hurt. I would not spend time around someone who was habitually hurtful to me. Actually, I have done that, and I look back and see what a waste of time it was.

You may figure out some intermediate way that allows you to discover more about her, without getting too over-involved.