I think his level of intelligence varies between continuities.
In the cartoon, he is honestly quite dumb while in the Marvel comics he could be perfectly intelligent, but was prone to rash decisions and pig-headedness. I think his speech impediment there was supposed to be some sort of viral glitch.
The Dreamwave comics seemed to start out with him in the cartoon's simple-minded persona, but from the second series onward he was more like his Marvel comics self. IDW also seems to have gone with the intelligent, but arrogant incarnation too.
Personally, I quite like both incarnations of Grimlock. His cartoon personality's one of the few things from the old cartoons that can still make me laugh, while I tend to find the smart, kinda brutal character more interesting to read about.