Well, many guys doing mocks are try to predict who the teams will draft, not necessarily who the best players are, so we have to define our terms.
But it would not surprise me one bit if you did a study and found that well-informed amateurs are at least as successful in picking the best players as the actual GMs. Studies in other fields, including medicine, have consistently shown that having more information available to you and more “training” isn’t necessarily better, so just saying that GMs have a ton of info available to them doesn’t answer the question.