Is it common idiom?
Yes, it is common in the UK.
Do you use it?
Yes, I do use it. I think it's funny and expressive. Imagine that you're trying to make a beautiful round pot on a potter's wheel. It's going fine, and then suddenly it sags at the bottom, and the pot is ruined. That's what 'pear-shaped' means.
Did you hear from others? / Do you hear other people saying it?
Yes, you often hear it.