Soto most likely includes the imagery of Alfonso's thumb in the passage to illustrate that Alfonso does not like getting himself dirty.
Explanation:
The author Gary Soto makes use of imagery in showing the wrinkled, tired and pinkish state of Alfonso's thumb to show that Alfonso does not like getting himself dirty.
The analogy used in the passage was that his thumbs had the appearance of someone who has been in the bathtub too long so we can conclude that the imagery is that of someone that does not like getting dirty.
