Simon can reasonably be said to have a 16, 17 or 18 Intelligence, depending on where you put Einstein.
But River is something more. She's almost supernaturally intelligent- all intelligence really represents is the capacity for learning, and River has superhuman learning ability.
I'd disagree with her low Wisdom, as well. She's not unwise...just unhinged. You could make a case that she has superhuman wisdom as well- and her 'madness', while extant, is also due to the fact that she's more aware of the world than most anyone should be. She only appears insane in some circumstances because she's so far beyond us.
Frankly,though, I'd say Unisystem Lite represents the setting and feel a bit better. All you'd really need to do is drop 'I think I'm Okay" and "Back from the Dead" from the roster of Drama Point options. Change up the skill list a bit, and whip up some Qualities and Drawbacks to taste, but D20 doesn't feel right.