What does the story of Estebanico tell us about race relations in colonial Mexico?

I assume that you are talking about the African slave who
is known by various names, including Esteban, Estebanico, and Estevanico.  He is the man
who was part of the Cabeza de Vaca expedition through North America.  I would argue that
he shows that blacks could, in very rare circumstances, gain some status, but that they
would always be seen as inferior in colonial
Mexico.

Estevanico gained what status he had through his
actions on the exploratory trip made by Narvaez and Cabeza de Vaca.  He was very useful
to the expedition when they were in extreme circumstances--when they were shipwrecked
and trying to make their way back to Mexico through unexplored North America.  In other
words, it took very unusual circumstances to allow Estevanico to get any
status.

Even though he had this status, he was not freed. 
When the expedition made it to Mexico, Estevanico was not freed or honored in any way. 
Instead, he was sold (or given) to the Viceroy.

This shows
that even if a black person could gain some through success in extreme circumstances, he
could not become anywhere near the equal of a Spaniard.