Mega Venu is in a very... weird spot right now. Someone earlier put it perfectly, but I can't look right now. Basically, half the new Megas (Gallade, Metagross, Mence, Lati@s) make it's life miserable, but on the other hand it handles Swampert, Sceptile, Altaria, Diancie, and Lopunny well in one slot.
Aragorn the King I do think that Mega Mence has a direct impact on Mega Venusaur's usage. It's tough to predict what might fill the usage gap that Mence creates, but Gallade won't see nearly the same usage and the Latis were already a problem for it and they aren't going to become more of one. Being weak to and naturally outsped by one of the most OP things OU has every seen is very bad for it's usage. I'd bet it's usage stats rebound after the Mence ban, because Mega Venusaur is the biggest loser in the Mence meta by a distance.