"Why does the narrator "always despised [Mahoney] a little?"
To the narrator, Mahoney represents the society that he has no choice but to associate with, but resents because he (and society) stifles his desire to express himself genuinely. We can see that the narrator has very introverted sensitivities and 'effeminate' tendencies than his friend with whom he's committed truancy which we can see in the following passage. "The reluctant Indians who were afraid to seem studious or lacking in robustness, I was one." (pg12 lines 23-25) We can sense that the narrator is stifled through the need he feels to conform to the normative characteristics of a typical young man. Those things which the narrator notices are peculiar, for a boy his age. Joyce contrasts his sensitivity as he notices the stranger's eyes, while Mahoney chases a cat up a wall. Similarly, Mahoney is disinterested in the stranger and dismisses him as a "queer old josser" while the narrator is interested in observing his character and even sensitive even to the fact that "his accent was good." Seeming a mere vagrant, the narrator alludes to the fact that this man must've been well educated as a younger man, but for some reason fell in class status, due to his own peculiarity and potential homosexuality and pedophilia.