As of close of Endgame, no-one (even herself) has referred to her as "Captain Marvel".
In Captain Marvel, she is referred to as "Vers", "Carol" and "Carol Danvers".
In Endgame she's barely referenced and then only as "Carol".
The trailer for "Spider-Man: Far From Home" does have Peter Parker calling her "Captain Marvel" but it's not yet known whether this is her "official in-universe" name or just Peter getting confused and making one up.
It's unlikely that we'll get a proper answer to this "proper name" issue
--until Phase 4 and/or Captain Marvel 2"