In "The Devil and Tom Walker," is Irving implying that there are degrees of sinfulness?
There is some suggestion of this, yes. On the whole the tale is told in a quite light-hearted vein, with the description of Tom Walker's cantankerous ways and especially his constant conflict with his wife who, like Dame Van Winkle, is presented as nothing more or nothing less than a 'termagant', a fiendish nag of a woman. In fact, her presence is so intolerable to Tom that when the devil carries her off Tom is greatly relieved. Similarly the devil himself is generally more comic than frightening, certainly in the earlier stages when he appears as a surly old woodman, and Tom's compact with him is not presented in a fearsome light either, but more in the manner of a hard-headed business deal.
Tom's initial striking of a bargain with the devil, then, is not really presented as a terrible sin. He is portrayed as a rather unpleasant character from the first, but his initial desire to get rich is one that is common enough to many people, and there is no real suggestion that he is committing a great evil at this point; the desire for wealth is not condemned out of hand in itself. What is a far bigger sin is the way that he goes about getting rich. He rejects the devil's first proposal that he become a slave-trader but agrees readily enough to become a moneylender, or usurer. Although not explicitly stated, the story undoubtedly implies that this is a great wrongdoing on Tom's part. He makes money through making others suffer, and still he remains vain and miserly in his ways:
He built himself, as usual, a vast house, out of ostentation; but left the greater part of it unfinished and unfurnished out of parsimony. He even set up a carriage in the fullness of his vain glory, though he nearly starved the horses which drew it; and as the ungreased wheels groaned and screeched on the axle trees, you would have thought you heard the souls of the poor debtors he was squeezing.
Apart from in the concluding moral, this is the closest the story gets to an outright condemnation of Tom’s behaviour, with the reference to the ‘poor debtors’ whose ‘souls’ he torments, without any remorse. Even when he does take up religion in later life, piously going to church, this is out of a purely selfish desire to salvage his own soul from the devil, not because of pity for his victims. The story suggests then, that making others suffer purely for one’s own material gain is really the worst kind of sin, far worse than the simple greed that Tom initially displays. He compounds his original greed by a continuing meanness of spirit even when he becomes wealthy, and by his hypocrisy and his lies. All of this adds up to quite a fearful aggregate of sins, and so, most fittingly, the devil finally appears to take him away.