Lack of attention from the people around, lack of organization - e.g. people who don't know the choreography
Nej
if it does not work
too much random chaos
Short and lame
if inconsistent to the culture and life style
When non-participants find it awkward, disturbing, weird, etc. Also when there are only 3 people in the "mob". When the flashmob has a message that is extremely controverse. When it offends or hurts others.
If it is predictable, unsynchronized dancing or just simply bad
One that scares/offends (There was a flash mob event with actors falling over from "heart attack" - I find that tasteless, even though the point was to inform)
Obvious marketing attempts, overthought with the hope of it going viral.