The joke is a pun.  A ventriloquist is a performer who makes it appear as if their puppets have a voice of their own.  Sometimes the puppets are referred to as 'dummies'.  'Dummy' is another word for stupid or unintelligent.  The phrase 'just a bunch of dummies' sounds as if the speaker is saying they aren't smart, but they might literally be the dummies the ventriloquist performs with.
Did you see guys that Bob the ventriloquist hangs out with ? They ' re nothing special , just a bunch of dummies .