Chekhov's characters are somber, but his own definition and intent here was indeed to write a comedy. Though not comedic in the traditional sense (there's no slap stick, practical joking, or romantic glitches portrayed here), what Chekhov gives us is a "tragicomedy." He uses sarcasm and innuendo to show us how his characters are tragic in context of experiences, and yet fully responsible for their own miseries because they've created those miseries all by themselves. This is why he pokes fun at them, and if you pay close attention you'll pick up on the humor, dark as it is!