G.K. Chesterton:

"IF A THING IS WORTH DOING, IT IS WORTH DOING BADLY."

Chesterton did not mean that the goal in our endeavors is to aim for mediocrity, 

but rather that we should not let the fear of mediocrity 

keep us from doing something worthy:

“The line, “if a thing is worth doing, it is worth doing badly,” is not an excuse for poor efforts. It is perhaps an excuse for poor results. But our society is plagued by wanting good results with no efforts (or rather, with someone else’s efforts). We hire someone else to work for us, to play for us (that is, to entertain us), to think for us, and to raise our children for us. We have left “the things worth doing” to others, on the poor excuse that others might be able to do them better.”

— American Chesterton Society