It's not being funny, it's not psychological warfare, it's not your right because you won.
You're a bad sport who is trying to make the other team feel bad, and you are everything that is wrong with the MOBA community. Saying 'Good game' (GG) after every match isn't mandatory, but it's a nice gesture that show you had fun playing against your opponents. Saying GG EZ is just being a big jerk and shows how fragile your ego is.
I get so frustrated and ashamed when I see anyone, even on my team, be a dick and type it in allchat at the end or near the end of the game. It's rude and condescending and should be weeded out at every opportunity.
EDIT: There are people coming on here saying "Oh get over yourself OP you just have thin skin, just quit playing online games if you can't take trash talk'. I've had people cuss me out and call me racial slurs, people insult me in every way imaginable at this game. But that's just them being toxic, do I have thin skin for hearing that every time I play a game? Saying gg ez or behaving in that manner is just being downright childish and unsportsmanlike and should be punished.