Best reason, whether intentionally funny or not. My rule of thumb is if it makes me laugh, I click funny.
Further, I don't regard it as a "derogatory" rating. It's usually evident when the reviewer is intending humor, and when they are not. Clicking "funny" on an unintentionally funny review isn't derogatory, its the RIA equivalent of diminishing an argument during debate.
In my opinion, there are some folks on here who take their own opinions way too seriously.
All of us, at some time or another, are horses asses, with horses asses opinions. Its up to the rest of the world to point it out to us.