We might come across someone who is mediocre at engineering but is an effective public speaker.
Someone who is mediocre at public speaking, but is an engaging content writer.
Someone who is mediocre at content writing but is an exceptional content editor.

People aren’t mediocre.
They only appear to be so in certain circumstances.
It’s often about the right person being at a not-so-right place.

Mediocrity is not an absolute. 
Someone who may appear mediocre in one world, might be a winner in another.