Drop Sharpedo out of C+...
Okay, so Sharpedo's niche is speed boost sweeping. Right away, competition from a more versatile and devastating Scolilipede. Both (Sharp and Scol) are easily revenged by Talonflame at any boost. Keldeo hard walls this guy, and multiple checks exist for the EQ/Crunch/waterfall variant including Azumarill, Breloom, Conkeldurr, Gyarados (-m) and Rotom-w. After rocks and 2 LO turns, Pinsir-m is an OHKO with Quick attack.
Stall doesn't even notice Sharpedo. Venu, Vaporeon, Amoongus, Ferrothorn, Quagsire, Gyarados, Skarm, Chesnaught, Clefable and even chansey avoid the 2hko.
It isn't the only "Fast cleaner". In fact, Sharpedo is really slow to get there, considering Swift Swim and Sand Rush mons like Kabutops, Excadrill and Kingdra all perform this role with more efficiency and are easier to make teams for. None, including sharpedo, are easy to put on any team.
And worst of all, it has competition for the slot by typing. Gyarados-m is far better. Crawdaunt is way more powerful, has boosting moves and priority. Greninja is a better speed/revenge killer.
Compare this to the mons of C+.
Aggron-m is the single most bulky physical wall in the game, and tanks most of the physical meta. A recovery issue holds it back.
Alakazam is a great revenge killer in OU, with incredible versatility. While Talonflame does this job, too, alakazam has great fire power and is a fantastic sash user given his magic guard ability.
Blastoise-mega is the best rapid spinner, but costs you a mega spot. He beats every single spin blocker and quite a few setters.
Entei is a great wall breaker this generation and nigh impossible for any non-fire physical mon to switch into given Sacred Fire. His choice band set supports good speed, a decent movepool and a niche nothing in OU does... Heatran can come close but 50% is better than 30% and the damage difference is real.
Ditto is the all-purpose revenge killer. It takes just about anything and kills it on a check, including megas. Because of this, it can reverse sweep. The presence of a ditto alone stops opponents from boosting.
In C
Dugtrio is an excellent mon that can revenge kill ZardX, Heatran, TTar and Excadrill lacking balloon without letting them escape. It can take down Charziard-Y, Pinsir-m or at least force more rock damage on them when they come in. It can beat Gyarados-m given good prediction and reversal. Aegislash staying in loses. Many other mons know a similar story when taking on Dugtrio. Of course, the lack of bulk makes this generally a one time thing but that's still a great trade for many teams.
Magnezone is a great partner for Pinsir-mega, allowing it to move past Skarmory, who otherwise is a huge pain in the ass. With the rise of Ferrothorn again, as well as the occasional aggro-mega (rarely carries EQ to my understanding), Magnezone gains more of a niche game to game. The attack isn't bad, either. Restricted by the 4x weakness to ground in an EQ heavy meta, otherwise very solid.
I'm skipping
P2 but you know how I feel about that one. Solid as any mon in C.
Blissey, for all the competition from chansey, is still bulky as hell and second best bulk cleric. It has a hard time differentiating itself but has a much better special attack and doesn't care about residual as much.
Wobbuffett... The bulky trapper, when used right, gains two or three kills. Of course, it has a dark weakness in a knock-off heavy meta, and can't really stop toxics but can be paired to do a tickle-pursuit combo to take out mons that choose not to attack.
These mons all have clear viability and clear uses. They may not be the best but used right, they're still very good. They're outclassed by one or two mons perhaps, not upwards of 10.
C-
Celebi: 4x u-turn weak, dark weak, fire weak, ice weak... aka terrible typing. good status absorber, awesome move pool... I wouldn't use in OU unless I was pressed to do so. It has uses but I can think of many other mons I'd rather use.
Reuniculus: Once feared, it gets hit hard by dark moves and can be revenged by all the powerful priority even when under trick room. Could easily use it, needs a great deal of team support. Might need to consider moving this up to C.
Gastrodon: Has like one niche in countering rain teams and rotom-wash. Tons of other water/ground types better for stall/bulky offense/rain (Quag/Swamp/Seis respectively) but the storm drain + reliable recovery is worth something.
Slowking: Is actually really good with that set
alexwolf has (I've been using it more often now because of that great special bulk) but still pretty bad vs the physical meta. Good special attack, has a bit of a hard time justifying itself compared to slowbro.
I think Sharpedo fits in with the C- rank way more than anything in C+...