Question

why did the post office employees make fun of ali

Answer 1

The post office became his place of pilgrimage.
The post office employees were very insensitive towards him . They made fun of him. They even began to call him a mad man . but Ali didn't react . He kept patience all the time for his daughter letter.

Answer 2

Because Ali went to the post office early in the morning for last five years  .so,  the member of post office thought that he was no work and called them a pest.  And also they used to make fun of ali