But if the usage stats are the only thing we're going off of, shouldn't bad pokemon that get tons of use like cloyster be in OU?
Either way it's a silly thing to go off of, they obviously wouldn't be overpowered in lower tiers without their mevos. All it would do is allow some more variety in those tiers.
Usage is the easiest and most reliable way to tier pokemon, assuming, as Dygenguar said, it goes off the usage of more experienced players. It runs on different logic than Ubers does, because Ubers (even though it has its own metagame) is basically just a banlist for OU and so does not run off usage stats. Basically the usage-based logic is, experienced players will naturally gravitate toward the better pokemon. If a pokemon gets used a lot, it may move up a tier. If it gets used hardly at all, it drops down a tier. If it gets used a certain amount and stays in a certain tier, but is too OP for that tier, it goes to that tier's ban list, i.e. Ubers for OU, BL for UU, BL2 for RU, and BL3 for NU.
The only alternative is to make subjective judgments for every pokemon about which ones should be where, which is much more difficult and imprecise than just being able to vote on whether "this pokemon is too powerful for this tier." Perhaps with something like vanilla Charizard it would be obvious that it does not belong in OU, but beyond that it gets murky so going off usage stats is a way to sort of "automate" that part.
Also, Cloyster is actually UU, though even there it's only used 5% of the time by 1760+ players. In OU, it's only used 2% of the time by 1760+ players. I.e., less than Donphan. Even across all OU players it only gets 4.6% usage.