We follow on-screen credits only when listing show stars. The Mandalorian only lists Pedro Pascal as a show star, then lists co-stars before listing guest stars. From season 3 Katee Sackhoff also is listed as a show star.
On TVMaze Co-stars are normally bit parts for example "Doctor" or "Nurse" and are not important characters that drive the show forwards. In the Star Wars shows this isn't the case, the characters listed as 'Co-stars' are much more important to the story and often can appear in multiple episodes. So because of this we have made the decision to list everyone but the starring roles as guest cast, unless the characters are defined otherwise (e.g. Special Guest Star) or not defined at all (Uncredited).