In the time of John and Jesus, the human dignity of servants excused them from undoing a master’s dirty sandals. That was one thing that the master had to do for himself. So, John, by saying he was nor worthy to undo the sandals of Jesus, was saying that compared to Jesus he was below the level of the lowest servant.
John’s memorable tribute to Jesus came in his saying, “He must increase, while I must decrease. It was his chosen life of penance in the desert, subsisting on locust and wild honey, that gave him that conquest of self love.