Crabrawler in C... I am sorry, but I have to disagree. Crabrawler isn't E material, but I do feel it works best in D. This thing... my god, this thing...
Crabrawler for D rank
Availability: On paper, you can find Crabrawler in the first berry tree you come across, and any others on all four islands... In practice, it's even more tedious to find than many rare fishing mons, because there's only a low chance of a berry tree having one there, and you gotta wait a day or so before it respawns. This means you have to be pretty lucky to find one on the first island, and camping for it is highly inefficient.
Stats: For a (potentially) early game mon, Crabrawler has pretty good stats. 82 attack and 63 speed are more than solid for the first island, but begin to falter by the third island. It's bulk is also sub-par, and while an Eviolite helps, you're going to be stuck with it for a LOOOONG time. As Crabominable, it gets even slower, but hits like a dump truck. Alas, this is horribly offset by...
Typing: Pure fighting is alright. Crabrawler packs a few decent resistances and a few annoying weaknesses, but it works... until Crabominable ruins everything by becoming Ice/Fighting, making it weak to a lot more stuff, and thanks to that slow speed and frailty, it WILL be hurting before it can fire off a hit.
Movepool: Crabrawler comes packing with Rock Smash, which if you got Iron Fist, is quite nice. Brick Break out the gate in the first trial makes Crabrawler pretty good... for the first trial, then things just start hating it. Lack of priority hurts, though it gets nice coverage in Crabhammer and Stone Edge and a few somewhat decent dark moves. As Crabominable, Ice Punch is solid, as is Ice Hammer from move tutor... too bad both of those are at the end of the game.
Major Battles:
Totem Normal: Raticate gets one-shot. Gumshoos MIGHT live a hit and call for help before dying.
Hau: Pikachu is going to be hitting you hard with Electroball, and as a Raichu, don't even bother. Eeveelutions are also trouble due to their bulk, and until you get Crabhammer, even the fire starter is going to give you trouble
Hala: Crabrawler is going down before it can get through all three mons, especially Hala's own Crabrawler.
Gladion: Zubat is NO, Type Null is doable, but Crabrawler is going to be hurting, Sneasel is doable, and Lucario... be careful. Silvally's type change hurts Crabrawler more than helps it out, and it won't be enjoying the pain.
Totem Wishiwashi: NO
Totem Salazzle: NO
Totem Lurantis: NO
Plumeria: NO
Olivia: Okay, this is doable, but if Nosepass paralyzes you, you're going to have a bad time. Also, Boldore lowering your accuracy sucks.
Molayne: Only Dugtrio is doable. The others are NO.
Totem Vikavolt: Crabrawler is not breaking through this by himself, and Spark hurts too much.
Guzma: Golisopod's gonna make the initial cut, which kills the crab instantly. Later on, Masquerain singly handedly ruins any attempt to solo Guzma.
Totem Mimikyu: NO
Team Skull: Crabrawler does... okay here, but any Golbats and Haunters are dangerous to deal with.
Nanu: On paper, this is Crabrawler's fight. In practice, you do squat to Sableye, Krokorok cripples your attack and nukes you hard with EQ, and Persian... You CAN beat it with Crabrawler, but only if Crabrawler's healthy.
Team Aether: Psychic types galore= dead crab. Most everything else tends to be too bulky for Crabrawler to reliably handle.
Lusamine: Not a single good matchup here. All her mons rip the crab apart.
Hapu: If the previous fights didn't cement how bad Crabrawler's bulk and offenses had become, this fight will. Crabrawler can only break through Dugtrio, and that's after taking heavy damage. Everything else is NO.
Totem Kommo-o: RIP crab, outbulked and outmuscled again.
Elite 4: Finally, Crabominable gets to take center stage... and proceed to suck horribly. Hala obliterates you, Kahili obliterates you, and you are not soloing either Olivia or Acerola (Though Crabominable does get to wreck Passoland and a few of Olivia's mons one on one.)
Kukui: Crabominable's only good matchups arguably are Snorlax, Decidueye, and Lycanroc, All of which hit you hard anyways. Otherwise, Braviary nukes you, Magnezone nukes you, Ninetails nukes you, and two of the three starters nuke you.
Additional comments: Crabrawler's biggest crippling weakness is the fact its evolution is locked right at the gates of the Elite 4. If you could access it before that point, arguably around the middle of the third island when you do pass by there, I could see it rising up to C or even B rank with the sheer firepower it gets, but because of the Mount Lanikaka evolution requirements, it's only really good at the start of the game... and then just keeps faltering and stumbling into limbo, and by the time it finally evolves, it's too late to salvage.