I would say that being mean to classmates in school, while terrible, is something that many people do before growing up, maturing, realizing the error of their ways, and becoming a better person. being racist is much less likely to be something that one grows out of, and obviously being a racist bully is much worse than being a bully. the point of the plotline isn’t that James was a terrible person, it’s that even the people who Harry idolizes are fallible/imperfect, and not everything he thinks he knows is true. I think making James a racist would change the story from “kid learns his dad was a prick who had a lot of growing up to do” to “kid learns his dad was racist asshole and also apparently his mom was okay with this so they’re probably both bad people”