>>15583504don't wanna open the can of worms, but that isn't the full extent of how tiers work either...
tiers are separated by perceived power and usage... which is to say the two are the same or one leads to another. Strong pokemon become OU because people will use strong pokemon more to win, which fills both criteria. the same isn't true if a pokemon just fits just the usage criteria, which is shown through ninjask, a largely used pokemon in OU for baton passing, except still an NU pokemon because its power is perceived to be of NU.
The same is true to an extent with ditto, as it is only as strong as the pokemon it transforms into, it fits into each tier, as the strongest pokemon it is against are those of the tier making it perfectly balanced.
When considering a competitive metagame, you have to take into account the correlation between usage and power. any pokemon largely used in UU is definitely used because it is a strong pokemon in UU... if it was too strong, it would be shown by it being used too much (over-centralization which smogon wants to stay away from) thus would probably be put into OU.