There is a phrase "What an idiot!". How should it be used for a group of people? Looks like "What idiots" doesn't have same emotional meaning.
You might consider referring to the specific type of idiocy:
"What a bunch of lemmings" (referring to a long-disproved misconception): a group of people doing something self-destructive, apparently of their own free will.
"What a bunch of sheep": a group of people accepting exactly what they are told by someone else, without thought.
(I'm not sure why the insulting version of these phrases, and Mark's suggestions of "a bunch of idiots", uses "bunch", rather than some other collective noun, such as "a flock of sheep" or "a herd of sheep" or "a colony of lemmings").