Don John is delightful because of his simplicity. He is pissed off at the world because his social status has left him feeling impotent, so he is just going to try to spend his time making other people unhappy. Misery loves company. We all know people who behave the same way, they just can't allow others to be happy around them and find little ways of undermining joy. I would imagine we all have relatives that way! So D.J. is a recognizable and annoying - but not destructive villian - that the audience can simply roll their eyes at without being distracted from the more entertaining love stories.
Side note: I agree it is a matinee smile, but I think B and B have much more complexity than many of the comedy characters. I also think there is an underlying depth that gives it an edge. Consider the last lines, when D.J. is reported "turned in flight." Has he been captured, or is he returning to fight? Will there be more war? Critics disagree, and the ambiguity lends a seriousness to the whole.