Is Garchomp really outsped by Pyroar? I think that's pretty variant on team size and level. You should at least have a decent amount of Speed EVs from random stuff throughout the game, too.
Anyway, I thought this was worth doing.
Trapinch: - D Tier
Availability: Route 13, one of the first Mountain Kalos Pokemon. This is pretty close to the mid-game but it really misses out on contributing for a while. It joins from level 26 to 28, which is a bit behind for this point in the game.
Stats: Trapinch has great Attack, but its other stats are poor and its Speed is atrocious. Vibrava isn't much better- it actually loses a good amount of attack, though it gains a significant boost in speed, while other stats only increase a bit. Flygon returns to having great Attack paired with good Speed and otherwise decent stats in other areas.
Typing: Trapinch starts off mono-ground, but Vibrava and Flygon are Ground-Dragon with Levitate. This is an excellent attack pair, but it really can't make use of it until it evolves to Flygon- more on this in movepool. Defensively, as Trapinch it's unfavorable due to weaknesses to Water and Grass, but fortunately it's not that far from Ground/Dragon, which is pretty good. Dragon attacks are rare while Ice is usually uncommon outside Wulfric's gym, though .Fairy can hurt. However it has several handy resistances, and both Electric and Ground immunities are excellent.
Movepool: Trapinch starts with Bulldoze but has Dig for a Ground-type option almost immediately (1-3 levels after it can be caught), which is reasonably powerful for a good while, and always has Rock Slide upon capture, which gives it good coverage. After that it gets Crunch at level 34 which can be pretty nice. Unfortunately, from here on it's pretty barren. The first real Dragon STAB it gets is Dragon Tail upon evolution into Flygon, as Dragonbreath is laughable coming off of base 50 special attack, and Dragon Tail honestly isn't much better due to negative priority. Flygon at least gets Dragon Claw at level 55, but this is fairly late and the TM isn't available until Victory Road. Flygon gets a fairly decent TM movepool while the other two have few options available. Interestingly, Trapinch gets Superpower through the move tutor while its evolutions don't. However, holding off on evolving Trapinch for that long is a dreadful idea.
Major Battles: Trapinch joins just in time for Clemont's gym, where it should do fairly well, though being outsped constantly is unfortunate and it might take a lot more damage than you'd like due to being underleveled. Vibrava can do some damage to Mawile but should avoid Valerie completely otherwise, as it really can't handle Fairy attacks. Olympia is a somewhat good matchup because you do have Crunch, but if you aren't evolved into Flygon yet it will probably have a hard time taking her heavy hits. Its performance against Team Flare is pretty good as it fairs well against Poison and Fire types while being alright against Darks. Lysandre will probably give you a bit of trouble but it can hit his team for good damage outside of Gyarados, which probably smashes you. Wulfric is a definite no-go. Malva is up in the air due to some neutral hits, but Flygon can at least deal heavy damage. It can also do well against Wikstrom. Flygon really struggles against Siebold and Drasna, however. Flygon can do okay against Diantha, probably handling the fossils on its own. It can probably do well against Hawluch and Gourgeist, but Goodra is too bulky for it to handle and Mega Gardevoir destroys it.
Additional Comments: The worst part by far about Trapinch is that getting Flygon is a real chore. Once you do get there, of course, you have a respectable team member, but 17 levels with bad stats all around is really hard to cope with. I just don't think it can justify the effort getting out from it's start in an efficient playthrough, and even as Flygon it can sometimes fall too short for me to really think it deserves higher than this. I just never really felt it pulled its weight.
EDIT: Didn't see Banryu's post but yeah, really disagree on B tier. I could see C at the very most. I don't have any Garchomp experience to compare it to though, so I won't say anything on that front.