There is a semifactual canon answer to this actually. Here are the stats of an average reaver as described by the core rulebook for the Serenity RPG.
According to the Serenety RPG gamebook info on Reavers:
REAVER
Agility. . . . . d8
Strength . . . . d10
Vitality . . . . d10
Alertness . … . d8
Intelligence . . d6
Willpower . … . d10
Life Points … . 24
Initiative . . . d8 + d8
Unarmed Combat d6/Brawling d10.
Discipline d6
Jayne, by comparison has:
Agility. . . . . d6
Strength . . . . d10
Vitality . . . . d10
Alertness . … . d6
Intelligence . . d4
Willpower . … . d6
Life Points … . 18
Initiative . . . d6 + d6
Unarmed Combat d6/Brawling d8.
Discipline d4
So they are smarter and more diciplined than Jayne (who knew!).
Mal however is twice as smart as Jayne, so smarter than a reaver and equally disciplined. Sounds fairly balanced to me.
To be honest, I think a combination of willpower, intelligence and discipline is what would count as smart/advanced.