How did Dr. Doom and Mr. Fantastic end up super smart?

Exceptional from birth. I suspect “super smart” is genetically coded.

Them being successful in economic pursuits is likely a factor of good education system and friend network. Hard work is something people who have had a good education and friend network say to explain their success.

From birth, but they certainly worked hard and went through university.

The Marvel universe celebrates quite a few super-smart science types.

Tony Stark (futurist)
Hank Pym (robotics & ai)
Hank McCoy (geneticist)
Bruce Banner (radiation)
Peter Parker (chemistry)
... and more, including many villains such as Doom.

Bonus points if you can name the above folks super hero identities.

In @cprevite‘s answer, you will notice how several of their super-smarts resulted in their accidentally acquiring super powers, both a gift and a curse.