What is the opposite word (or antonym) for the word "Behold" as in the phrase "Behold the mighty ..."?
Behold is defined as
however it is an old word, which uses these meaning specifically
So, the question is which meaning and usage you want to make the opposite of.
As Larry Morries suggested, one possibility is to ignore. Another would be your suggestion to miss (ignore denotes conscious effort, while miss does not).
Few other antonyms are: disregard, neglect, overlook.
Although each time you use behold there is a certain grandness and a sense of pause that is implied: 'Behold the mighty Joe!'
'Ignore the mighty Joe!' does not carry the same tone, but it is an antonym of 'to gaze upon' and it is an imperative.
Maybe to preserve the tone of the whole phrase: 'Pay no attention to the mighty Joe!' works better.
"Avert your eyes!" would specifically mean "Look away", with a purpose to avoid the physical or psychological damage of looking at welding sparks or a crumpled car with half a child inside.
"Move along folks, nothing to see here." Is traditionally accosted with a policeman or other first-responder trying to get a crowd who has gathered to observe some dramatic crime or accident. Whereas the former is an impassioned warning, this is purposely boring.
The most recognized example of an antonym to "Behold!" in American film would have to be "Pay no attention to the man behind the curtain!" from the Wizard of Oz.