Yes, Irving does imply that there are degrees of sinfulness. One of the more obvious examples of this implication is when the devil suggests to Tom that he make his fortune by slave-trading and Tom refuses. Tom is shown to be a heartless man (his only concern when his wife goes missing is the return of the valuables she took with her), but even he won't commit a sin that great. Earlier in the story, when the devil and Tom Walker first meet in the forest, Tom notes that some trees had been "more or less scored by the axe". This seems to indicate that perhaps the greater the score by the axe, the greater the sinner.