Does a countered Mimeoplasm still exile creatures?
The wording clearly says


  As The Mimeoplasm enters the battlefield, you may exile two creature cards from graveyards. If you do, it enters the battlefield as a copy of one of those cards with a number of additional +1/+1 counters on it equal to the power of the other card.


(emphasis mine)

So if it's countered, it will not enter the battlefield, the replacement effect won't happen and the creature cards won't be exiled from their graveyards.