Mr. Magniloquent
Arcane
Also remember the dialogue where one of the party members (Jaheira, I think) acts as a stand-in for some of the playerbase, and wanders aloud how can he be considered a ranger and a servant to Nature, rather than a brute fighter? To which a short scene is depicted where he treats squirrels in a very kind manner (I think he helps them collect nuts, or something along those lines)?
I think that Minsc is supposed to represent the idea that true goodness is simple and uncomplicated: a good-hearted retard is more of a hero than a thoughful, brooding anti-hero which were so popular at the time.
That and because they didn't have time to implement the barbarian class in BG1, so they made him a ranger. By the time they had the class in BG2, the precedent was established.