If original sin doesn’t exist exist, then sin is still absent and all things are good and there is no evil. Wait, that isn’t the case is it?
So sin is here, and renders us hell bound without redemption. So, yes the IC and Jesus DO matter. There is no relativism to it.
How do you know that isn’t the case? Perhaps a sense of good and evil is something God instills in each of us, in order for humans to create societies and function together. Maybe there is no good or evil to God?