Those Christians I know who believe the doctrines of grace don't view sin lightly. They realise that sin is so serious that God sent His Son to take on human flesh, and to die in order to save sinners.
They think they do not view sin lightly, and compared to many religions they do not. But in comparison to traditional Christianity they view sin very lightly.
We all agree that God sent His Son to take on human flesh, to come "in the likeness of sinful flesh", to be "made sin for us", to "share [our] humanity", to "share our infirmity". And yes, this is because of sin.
BUT prior to the Reformation the idea that divine justice could be met by God punishing our sins on Christ was foreign to our faith. Sin was viewed as much greater an offense, much larger a problem.
Viewing the remedy for sin as God punishing those sins on Christ is viewing sin very lightly in comparison to traditional Christianity. Viewing punishment as satisfying the demands of divine justice is minimizing divine justice.
Sin is much greater a problem than Calvinism believes, and God is more holy a God than their doctrine will allow. There is no getting around that when you view Calvinism alongside a traditional form of the faith. Calvinism is simply "easy believism".