Riot has some set "guidelines" to decide whether a champion is under performing or over performing.
Source: https://nexus.leagueoflegends.com/en-gb/2019/05/dev-champion-balance-framework/
Average Ban Rate is 7%.
#Average Play: Iron 4 -> Gold 1.
A champion is considered Overpowered in Average Play IF they meet one of the following conditions
- More than 54.5% win rate while being below the Average Ban Rate
- More than 52.5% win rate while being at 5x the Average Ban Rate or higher.
A champion is considered bottom end in Average Play IF
#Skilled Play: Platinum 4 -> Master tier.
A champion is considered Overpowered in Skilled Play IF they meet one of the following conditions
- More than 54% win rate while being below the Average Ban Rate
- More than 52% win rate while being at 5x the Average Ban Rate or higher.
A champion is considered bottom end in Skilled Play IF
#Elite Play: Grandmaster and Challenger tier.
A champion is considered Overpowered in Elite Play IF they meet one of the following conditions
- More than a 45% ban rate.
A champion is considered bottom end in Elite Play IF
- Their Presence is less than 5% (Presence is considered Pick and Ban rate combined)
#Professional Play: LEC, LCS, LCK etc
A champion is considered Overpowered in Professional Play IF they meet one of the following conditions
- Their Presence is 90% or above in the current patch or 80%+ over consecutive patches (Akali for example)
A champion is considered bottom end in Professional Play IF
- Their Presence is less than 5% (Presence is considered Pick and Ban rate combined)