As someone who has GMed for THUNDERLIPS, one of those situational modifiers is "break the GM." After I am done falling off my chair laughing, I usually give him a +2.
Making me as a GM laugh is not worth +2 situational on diplomacy. Actually being diplomatic will. Thunderlips has yet to ever earn a +2, and usually he's at -2 after he whips out his ... continual flame during a tense negotiation.