less intelligent (Ripcord; in the movie they specifically pointed out that Duke's test scores were higher, why I don't know),
that wasn't Rip's intelligence. that was his scores in combat tests. like the holographic targt range thingy. and i think the point they were making was Duke was f'ing badass and Ripcord was good, but not good enough to be Joe material. the Joes, it was emphasised were/are the best of the best. the alpha dogs. and clearly the Joes are not racist - note Heavy Duty, Breaker and the Asian chick in the invisibility suit - it's just that Rip, whilst good enough for the US Special Forces was not good enough for the Joes. they take the top 1% of candidates, he fell into the 2nd % of scores. Hawk basically let him in because he needed Duke's co-operation on finding the Baroness and the warheads and probably figured if he kicked Rip out, Duke would follow out of loyalty to his friend. possibly, there was also some sympathy to Rip and Duke's desire to get even for the attack on the convoy.
Rip, however, did prove his worth by helping to save the day. let's not forget he saved Moscow and Washington from the nano-mites.