As far as I know, petaQ is an insult, but the specific meaning has
been withheld, probably to make it safe to use in a program for
children.
As such, we cannot desume that using -mey is an additional insult; as
far as we know, a petaQ may be a disgusting animal, or thing, so -mey
would be the grammatical correct way to pluralize it.
Am I missing something here?

I agree. We just don't have any data on which to speculate. All we know is that in paq'batlh Molor calls a group of assembled Klingons petaQmey.

-- 
SuStel
http://trimboli.name