Even the worst mentor offers better training on how to inspect a house than a RE agent gets ! IF an appraiser has a weak mentor, they can supplement that with additional reading /other education on the subject.
Mainly though it is the years of field experience / inspections of many diverse properties that educates the appraiser and forms a cumulative knowledge base- as well as reconciling what they observe at the property with the cost approach, data analysis, comparative property ranking, talking with contractors, flippers , homeowners , builders, etc.