The young people the X-Men recruit are all mutants, born with inherent amazing mutant powers - but before they can be X-Men, they have to learn how to wield and control their powers, otherwise they are a danger to themselves and others. So they were a part of the New Mutants until they 'qualified'.
So, being born incredibly intelligent is of no use until you learn to control and apply your awesome ability, otherwise you're a danger to yourself, and probably others around you through your wild unfocused and oftentimes thoughtless actions.
Yes I'm a comic book nerd. :P
Not that I'm incredibly intelligent or anything. Easily amused, definitely.