I’m not sure he’s “underrated”, almost everything I’ve heard about him describes him as one of the most intelligent people to have ever lived. I have seen more than one article proposing cloning his DNA as the best way to save humanity
But, I've just learnt an anecdote about Neumann, Einstein and Godel.
When Neumann signed for Princeton, he had two conditions: a 16k annual salary (others earned ~2k at that time) and to bring in that two people who are smarter than he.
(Source, in Hungarian, someday we will have the subtitles from the AI: https://www.youtube.com/watch?v=pu6xoHlr0zk)
UPDATE: The talk is about miracles and geniuses. According to the speaker, a genius is somebody who sees something which wasn’t in the air, up until that time. Neumann wasn’t a genius, rather a polymath who saw all what was in the air at that time
I'm sure I will not find anything. But it would be interesting for people to analyze the outliers.