It may or may not be.
Usually US consulate checks if you have strong ties to canada like job, residence, basically they want to ensure that you don't move to usa.
They may or may not ask you to show your PR card. I don't remember them asking when i applied for my visa.
Also, expired pr card does not mean that you are no longer a pr however it may be a concern for them.
Did they ask you to bring PR card for interview?