I think that Ed is not brain damaged, in fact, I think he's smarter than the other hyenas. If you look and understand carefully, in the scene when Simba, Nala and Zazu escape from the hyenas when they are talking about 'Cub Sandwiches' and that weird things, Ed was the only one who notices it. He tried to tell Shenzi and Banzai that they were escaping from them, so he was smarter than Shenzi and Banzai by paying more attention to the cubs and the bird than to the hyenas saying stupid things. And when Mufasa busted them trying to eat the cubs and the bird, he was the only one who telled the truth, and always is smarter telling the truth than lying like Shenzi and Banzai.
Maybe Ed was borned with a damage on the tongue or the vocal chords that made him incapable to talk well to understand or vocalize (there's a guy in my class that has that problem) But It's very clear that he's very smart compared to other hyenas.