Although kind and principled, George's temper sometimes gets the better of him. The question here is whether this temper affects his judgement of the bus driver and others in the book.
As regards the bus driver, as George's anger builds, the distance the bus driver has left them to walk grows from 'damn near four miles' to 'more than four miles'. However, the precise distance is unimportant and George is right that it was a mean act to leave them to walk so far. No doubt George feels that this is typical of the shoddy treatment handed out to itinerant workers and therefore his condemnation of, and anger with, the bus driver is justified, his evaluation reliable.
Later we see this pattern repeated: Steinbeck describes George 'working up a slow anger' as he suspects that that their bunk house beds are infested. Once again his anger is justified and his assessment of things reliable. Itinerant workers did have to put up with notoriously poor living conditions and this is difficult for a proud man llike George to tolerate.
Throughout the book George proves to be reliable: even his harsh dismissal of Curley's Wife as 'jailbait' proves to be accurate in poor Lennie's case. His temper is rightminded frustration with a cruel world; it does not indicate that his judgement is unreliable.