It's sorta both. Here I'll get into it:
You have 3 tiers kinda close together below Ubers. OU is the standard Metagame and is everything minus Ubers. Right under that, you have UU which is full of Pokemon that work less efficiently than their OU counter parts basically (or they might have a noticeable typing flaw that gets to them).
However, inbetween OU and UU, you have the BL/Borderline tier. These Pokemon are too centralizing to the UU metagame, but they may not really be up to par for the OU meta. Listing them as OU material would be misleading, and almost always proves itself with Usage and win stats. Because of this, it tends to prove that they are statistically not useful enough to be labeled OU, and are basically put into BL for the sake of the UU metagame.
They do a ton of analysis every month on Usage and Win rates, and it's near impossible for one person (or even a select few) to shift a character's usage so significantly, like the Magikarp example. You have different sets of data as well, like using different Ratings as a baseline for what counts. Or top 100 on the ladder (assuming they play enough games), you can have their stats vs anyone over idk 1700 or something that would put you into the top few hundred. Or even drop further down if you wanted to. Usage, how often that Pokemon shows up on both teams at the same time, what sets are being used on Pokemon, and I'm pretty sure they also (if not recently) have certain win rates available too. Like say a top tier OU pokemon has roughly 16% usage overall, and not counting dittos wins 64%+ of the time. You can't "fake" usage and also be successful at those kinds of levels, certainly not as you progress the ladder.
With 4th and 5th gen, the overall "mark" to hit for OU usage usually hovers around the 50th mark. They calculate and hold stats, even for the random "16" people that used a really dumb UU/NU Pokemon on their team. Statistically, you drop into incredibly low usage %'s and win rates past that point of 50ish.
TL:DR
You can't get away with using a really crappy Pokemon, win a ton, and boost them up to OU alone. It tends to take multiple top players contributing to that Pokemon, and occasionally Pokemon who are more than capable of doing a specific role, who are already in OU, get glossed over for "worse" Pokemon that can stop very specific threats the person's team might struggle against a bit more than usual. The risk being if they don't run into those Pokemon, their "CP" Pokemon from UU tends to not be that useful and even further demonstrates itself in battles why it isn't really OU material. A perfect example would be Slowbro or Amoonguss for their specific walling capabilities vs specific threats. They clearly aren't OU material, but they can find minor niche's in the metagame and can have solid usage due to this.