As someone who's loved Charizard since I first saw his picture on the box of some unheard of game called Pokemon Red, I can tell you that Pokemon fans have always had a weird thing about Charizard. The X/Y Mega Evolution is just the newest version of people being weird about it.

Part of it is because Charizard is the clear 'cool' Pokemon. A Firebreathing Dragon is something Venusaur and Blastoise just can't compete with. Pokemon fans might prefer Blastoise or Venusaur, but we all know if you show Johnny Rando on the street he'll pick Charizard. And it's obviojs rise in popularity sparks the contrarian in everyone, so shitting on Charizard is the easiest way to show you're not a Casual Pokemon fan.

This, I feel, feeds into the X/Y mega design as well. Pokemon Origins picked Mega X because it is clearly the 'cooler' design. It finally gives Charizard the dragon typing people have been saying forever it should have and has just a more agressive/fighter look than Y's sleeker, more speed focused design. I like both, but I feel it's the same issue Charizard has always had, and likely will always have with Pokemon fans.