In Bull Durham, Kevin Costner's character may be a loser, but he isn't a dim bulb. He's as smart as Susan Sarandon's character (and their mutual recognition of the other's intelligence makes up much of the erotic charge between them), and not just because of his famous Susan-Sontag-Is-Overrated speech. What makes him poignant is that he's a smart man who knows he's a mediocre ballplayer, which is what makes his mentoring of the genuine dim-bulb (but great pitcher) played by Tim Robbins so funny and sad.

And in Tin Cup, he plays a genuinely great golfer who's simultaneously too proud and too much of a slacker to care about winning. As the ending makes clear, he'd rather hit the perfect shot than win the tournament. Again, kind of a loser, but not a dim bulb.

None of this is meant to say anything about Costner in real life, about which I know nothing. He could be as dumb as bag of hammers, for all I know, but at least in these two films, the characters he plays are not.