When characters make use of oxymorons, they are using arhetorical or literary device, like a metaphor or simile. They aredoing this to get their point across better. Whether they "overuse"them or not is a matter of taste, but if a lot of people think thatthe use of oxymorons was excessive, then they are not succeeding atgetting their point across better. If in the listener's mind theliterary devices get in the way of the point that is being made, itoften shows that the speaker is more interested in showing off hiscleverness than saying anything of value.