Mega Garchomp is not bad. Definitely not as bad as Mega Audino.
It’s known to be worse than standard Garchomp.
Garchomp is a Sweeper with good attack and decent speed. Mega Garchomp swaps it’s Speed for bulk. This makes it weak to fast moving sweepers as well as just plain slow, hence more people use standard Garchomp.
Mega Garchomp also doesn’t get a life orb which basically does the same job as Garchompite but with 10% HP recoil and no speed drop.
Mega Garchomps Ability is also much more situational and rough skin can end up activating a lot in battle so more people prefer Garchomp’s ability.
So MGarchomp doesn’t have the Speed,Items or ability to be as good as Garchomp.