Everyone realizes that Barnard doesn't have a large student body, so ethnic diversity in small numbers counts for a lot. However, there's not much political or religious diversity to be seen on campus, and one might argue that this is just as important as ethnic diversity when it comes to the enrichment of students' lives on campus.
The key reason for diversity on campus is that it brings groups together for different causes, like Republicans and Democrats conversing because of their religious or ethnic commonalities. Just like many other schools, Barnard has many people of different races and cultures, but for the most part, these people tend to stick together. It's great that there's diversity on campus, but there's no point in having it if no one mingles. It's really not fair that the school is looked down upon for this, when ultimately it's the responsibility of the students to get involved with diverse groups on campus. Barnard does try to foster diversity on campus, especially ethnic diversity, with organizations such as the McIntosh Activities Council's Multicultural Committee, which puts on the One World show each year. Overall, Barnard does try to be very diverse. It's just the students who don't cooperate that give the college a sort of clique-ish feel.