Who is Mickey Mouse in the movie A Time To Kill

It is indeed one of the local Klansmen (or at least someone from Freddy Lee Cobb's friend circle). It is John Diehl's character Tim Nunley. He is first seen at Willard's and Billy Ray's funeral when Freddy Lee and his friends first discuss getting the Klan's help, where he wears a blue overall and drives a red pickup truck. He has a full beard.

There are hints as to him being not entirely on-board with the Klan's more serious deeds. During the ceremony where they kidnapped Roark, you can repeatedly see the camera focusing on him being a little distraught by the action:

There is a clearer shot of him and his Mickey Mouse tattoo when he later comes to rescue Roark:

As to why he would do that when he's part of the Klan, we can only speculate. But it is not too far-fetched to assume that he just got dragged into this out of genuine anger over the death of two of his friends and also because of his affiliation with Freddy Lee. But when he later realizes what he got himself into and what the Klan is doing, his conscience prompts him to help those people and sabotage the clan.

The point is, with organizations like this, it's not too easy to just turn around 180° and say "Well, this lynching thing isn't really for me. I'll pass, have a nice day." He was too deep in (and might as well have felt loyalty to his friends).

Per IMDb:

The character with the Mickey Mouse tattoo who warned Jake's wife about the cross-burning and who released Ellen after the Klan tied her to a tree is named Tim Nunley (played by John Diehl) in the credits.

In the movie, he was simply a reluctant member of Freddy Lee's KKK group. Those who have read the novel report that Nunley was working as an informant for the police.