OP.GG says that half of the champ roster has a winrate below 50%

lmao

ideally, yes that is what is suppose to happen. The overpowered champions get picked more and are able to keep multiple lower tier champions at a lower winrate (i.e. 1 overpowered champion at 10% pick rate and 55% winrate will balance out 5 lesser champions at 2% pick rate and 49% winrate).

However, there are always factors such as high pick rates artificially deflating winrates which can make the data more confusing to interpret.