Most of the time, yes. The speed loss is further compounded by the fact that you cannot hold a mega stone. Mega Chomp may look awesome but that loss in speed really hurt. Being unable to outspeed Charizard and Salamence is a huge deal since that was one of the things that made Garchomp so good
It's ability, sand force, is also next to useless. Sure it boosts it's ground, rock and steel type moves but does it really need that much? I think no since the only time that would truly matter is against unaware Clefable. Plus, in sand, regular Garchomp is even better because it can threaten to end the game due to how annoying the rng is
The one saving grace of Mega Garchomp is that in gen six, mega evolution keeps the speed of the base Pokemon on the turn it mega evolves. That means, for one single turn, you get the best of both worlds, Garchomp's troll speed and it's mega form's massive attack stat
Make no mistake though, Mega Garchomp is still a very powerful Pokemon. The problem is unlike regular Garchomp, it isn't as easy to put in teams. In fact, I think the best way to use it is to build a team around it rather than slap it on teams. But the bottomline is, Mega Garchomp is one of those specialized Pokemon and just like in life, you're only gonna want something specialized when you need its uses
In gen seven, Mega Garchomp actually fares quite better. Since mega evolution speed mechanics change, it can afford not to go for a +speed nature. In fact, the four most dominant Pokemon, Landorus Therian, Heatran, Toxapex and Magearna are all mauled by it's mixed set, sometimes even outright killed in one shot if they've been chipped enough but still, it does not change the fact that it remains a specialized Pokemon
So, in short, Garchomp is a swiss army knife, meaning it's versatile and easy to slap on teams. It's Mega form is a kommando tri dagger I guess and is very specialized. The final answer to your original question is yes, Garchomp is better in its regular form simply because it's easier to put on teams and since it's not restricted by a mega stone, it can take full advantage of it's greatest strength, versatility
