Flying creatures can't be rendered prone
Pathfinder creative director James Jacobs addresses whether or not a flying creature can be rendered prone in this 2012 exchange:
Can a creature… with natural or magical flight be knocked prone?
Not when they're flying. You can mess with a creature in the ways detailed in the Fly skill, which might make them fall, but you can't make a flying creature prone. Or a swimming or burrowing or climbing creature, for that matter.
Absent this pronouncement, the GM determines the effect of rendering a flying a creature prone. Note that flying creatures were already immune to the trip combat maneuver (which, when successful, often results in a prone foe): "Some creatures—such as oozes, creatures without legs, and flying creatures—cannot be tripped."
Creatures that fly usually fall if they can't take actions
As per the FAQ and as cited in this fine answer to this earlier question and this fine answer, "any creature that loses all actions can’t take an action to attempt a Fly check to hover in place and thus automatically falls." This includes a creature that's flying without the use of wings (like a witch employing the hex flight to use an effect like the spell fly). The spell hideous laughter makes it so the subject can "take no actions" for the spell's duration, so a flying subject usually falls.