Ali is as lead as you can get. The entirety of the film revolves around his relationship with Mortensen, and while it's true that Tony Lip grows more as a character, it's because he has a larger gulf to span than Don Shirley, who nevertheless comes out of his shell and becomes more of a social character. If Ali were a woman, it wouldn't be up for debate.
Simmonds is co-lead with Krasinski and Blunt. She has as much perspective as them, and her character probably goes through the biggest arc of them all.
Ali is supporting. He is only there to guide Mortensen. His coming out of his shell a little is very much secondary to Mortensen going from throwing away glasses used by black men to inviting a black man to his family Christmas dinner.
If Ali were the lead, it would've been a much better film. The fact that he is treated like a supporting character when Don Shirley should be the one leading a biopic is a big part of why the film is kind of a slap in the face.