For a hiker, I'm worse than average :) Since the estimates in our mountain guides are 'for the average person'. Of course I'm better than someone who just walks to/from the car.
As anecdata I'm pretty sure I did a 3 hour estimate hike up a mountain in 1.5 to 2 hours when I was 19, enthusiastic and in top shape. So they really are averages.
There are plenty of people for whom mediocrity would be a big step up. If you have terrible health, having mediocre health sounds delightful. If your partner is abusive, having a mediocre one looks like a grace.
Your sample selection for "the entire world" seems to be leaving a lot of people out.
Nobody's saying you ought to hope for mediocrity. But plenty of people do.
In fact, for most things that aren't important to me, mediocrity is the level I hope to attain. Sure, I could probably achieve higher, but if it's something I don't value, putting in that additional effort would be at the cost of something I value more. If my achievement drops below mediocre, it's usually bad enough to cause problems, so mediocrity is my "not very good, but not bad enough to be a problem" level goal for things that don't matter to me.
Again, nobody in the entire world prays for a mediocre life, a mediocre health, a mediocre salary, a mediocre partner. If you could put in the same amount of effort and get better results you'd accept on the spot. There is joy in things, no matter how good you are at them, but being mediocre at something certainly doesn't bring more than being average or good at it.