DS9 cast members such as Michael Dorn, Armin Shimmerman and Nana Visitor have all commented in the past that the DS9 set WAS a much more subdued, 'serious' place. They took the work seriously, and tended not to have fun and hijinx akin to the TNG cast. They were also not all "best mates" like the TNG cast (Armin talks specifically about Avery Brooks being quite distant from most of the others on a personal level) but were always cordial to each other.
Now, that is not to say there was something "wrong" with the DS9 set - it's just a different way of working. As many of them came from theatre backgrounds, it's easy to understand this. In fact, you could argue they had the right approach! (Marina Sirtis often gleefully talks about an early TNG director who refused to come back on the show because the cast were so silly; now, she says it as a good thing, but from the other perspective, maybe the director felt they were unprofessional and ruining whatever he was trying to achieve).
As for not being best mates, well, I work with lots of people, and I wouldn't hang out with most of them - I don't have anything against them, but there's no law that says people you work with must also be close friends. That's not to say some strong relationships didn't develop amongst the ensemble - Brooks/Lofton, Siddig/Robinson, Siddig/Visitor (obiously).
Dave Rossi, who worked behind the scenes, has also gone on record (in the UK Star Trek Monthly magazine back in 2006) stating that the behind the scenes atmosphere on the show circa season 2-3 was very low. They felt overshadowed by TNG and VOY and many of the cast were fed up.
I rather suspect most of the behind the scenes of TV series are a mixture of good, bad and ugly!!