I don't think Doran is dumb. I think we have more to see from him. A lot more. GRRM said that Doran "plays to win", for whatever that's worth.
And he might not have achieved much as far as revenge goes, but other than Oberyn(who got himself killed), not a single drop of Dornish blood was spilled in the War of 5 Kings. I'd say that's a pretty big acomplishment.
As for Quentyn, many people complained, but I liked his arc. He was a deconstruction of the typical fantasy "hero on a noble quest" trope. Much like Sandor is meant to dispel the romantic notions of knighthood, Quentyn is meant to dispel the romantic notions of being a hero. The point was that in 'real life' that's how quests like his end up more often than not, in tragedy.
And Quentyn was not dumb either. Just a dutiful boy who was handed a burden that was too large bear. It can't be easy knowing that the fate of your family and your entire people rests on your shoulders. In the end he tried his best, and came up short.
His death was sad, but in a comical sort of way:
"All of him, all of him was burning. Oh he thought. Then he started to scream"
GRRM is a fool for that one.