John Gill Commentary Psalms 69:11

"When I made sackcloth my clothing, I became a byword unto them." — Psalms 69:11 (ASV)

I made sackcloth also my garment
Though we nowhere read that Jesus put on sackcloth upon any occasion, yet it is not improbable that he did; besides, the phrase may only intend that he mourned and sorrowed at certain times, as persons do when they put on sackcloth: moreover, as the common garb of his forerunner was raiment of camels' hair, with a leathern girdle; so it is very likely his own was very mean, suitable to his condition; who, though he was rich, for our sakes became poor;

and I became a proverb to them ;
a byword; so that when they saw any person in sackcloth, or in vile raiment, behold such an one looks like Jesus of Nazareth.