Big Mac communicated just as much with his "Eeeyup"s and his "Nnnope"s as anyone did with full lines of dialogue. He's always been a pony of few words, and I think having him launch into full dialogue has to be used very sparingly, like Ponyville Confidential did.
In as far as the joke made at the end of the episode, I did say "Dammit Grannie Smith!", but you know what? I laughed while saying it. It was funny.
It takes a good writer and a good actor to make a character say a lot with just one or two words.