Mater Dei is the primary feeder (at least it used to be), but kids come from all over and bond with one another pretty quickly freshman year. If anything, boarding students are the ones who complain about being socially excluded from day students. Geography of day students can make a little difference. Population from Potomac, Chevy Chase, Bethesda outweigh NW DC, NW DC outweighs suburban VA.
And yes, non athletes get plenty of respect. The President of the Yard (i.e., prez of student body voted by students) is most often a non athlete. My senior year, the POY was also the valedictorian – asian minority, zero sports (not even club fencing

), not one to socialize on the weekends, and from (gasp) Wheaton. He clearly fit in fine.
But the culture values well-roundedness (academics, social, athletics) above all else. I think that's typical in any school. Admissions looks for balance – kids who are at least proficient in all key areas but also REALLY excel at one or two things be it lacrosse, piano, baseball, or mathematics, etc.