All sin is wrong, but not all sin is mortal. The Bible plainly tells us so:
1John.5[16] If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. [17] All wrongdoing is sin, but there is sin which is not mortal.
Hence there are sins that are mortal. Sins against the cardinal virtues (faith, hope, and charity) are mortal, as are sins against the 10 Commandments.
Venial sins block our relationship with God while mortal sin kills it. Venial sin can certainly lead to mortal sin, but they are not mortal.