You do not have to be the very best in a sport to be an excellent coach. However, you do need to have mastered the game, IMO. Mark Dvoretsky is an IM and generally acknowledged as one of the best chess coaches in the world.
He is, however, an IM. It takes years of experience to reach that level. Plus it requires norms, not just rating points, to qualify for the title.
He himself had great chess coaches.
I think it is extremely doubtful that there are any successful tennis professionals whose coaches were themselves just mediocre players of the game.
The more I study chess and the more I believe I know about chess, the more I realize just how much more there is to learn and how much I do not know.
Well what were you expecting, some meaning ful analysis of your games?
lol true