Simply not having an interest in science or math already makes you a B student in those areas, since no matter how good you may be at them, eventually you'll hit a breaking point where the material is much harder than you can manage without having already practiced and honed all the previous material to a certain level of mastery.

I think the term "B-student" is just an effective way to identify to the reader students who wouldn't do well in math or science for any reason, whether environmental, motivational, or natural. That is, it brings about a connotation of someone who is smart enough to succeed at most things, but who can't compete head to head with students who specialize in a specific academic field.

It's hard to define, but the term's usefulness comes from its general connotation and the associated imagery, not its definition.

I think the takeaway here is that these individuals, whatever you call them, should focus more on learning skills that will allow them to compete laterally by creating opportunities for themselves and others rather than learning to be superficially cultured or trying compete for existing opportunities head to head with others who are more specialized and talented in their respective field.