I VERY strongly disagree! Yes it's true that megas added to the competitive scene, but thats about it. If you play pokemon to collect the ones you think look the coolest, then mega evolution is... Blech...
While mega quaza's design does have some likable qualities, overall his design is, you guessed it, over the top... I'm gonna go ahead and prove your point wrong by saying that I, do in fact, dislike mega rayquaza's design, so that argument is invalid right off the bat there.
The reason he's effective in every way he's used is because he is so obscenely overpowered, that it is nearly impossible to lose with him. He even got a tier of his own and even has a higher BST than arceus. He is so absurdly powerful, that he is even stronger than literally god itself.
Yes you could argue that megaquaza is awesome, but that is simply your opinion. I personally dislike mega rayquaza. If you like mega rayquaza, then good for you, but don't start throwing around insults about my favorite legendary. People on the internet aren't exactly going to like you for doing that, or take you seriously, and I certainly don't either.
How would you feel if you had a legendary pokemon that was your number one favorite for years, then suddenly got a mega evolution you despised? That that mega sort of ruined the actual pokemon for you, but you still liked the actual pokemon? But none of your friends gave a shit about the actual pokemon, devoted all of their attention towards it's mega evolution instead, and even resorted to bashing the non-mega form when you disagreed?