I'm very torn on this film.
If I had to pick one significant flaw, it'd have to be Lee's decision to cast himself in the role of Mookie.
The dialogue, actions and ideology of the director's character can sometimes be seen as a refraction of the director's own characteristics. It is not unlike what Jean Renoir did with Rules of the Game (1939).
I think it's incredibly important for Spike Lee to place himself in the movie. This suggests that it is not only up to Mooky, but also Spike Lee to actually do the right thing. This is part of the reason that this movie got everyone thinking, even after the show was over.
So in my opinion, the bad acting isnt so bothersome considering what it means to me on these other levels.
...one thing I really admire about Do the Right Thing is its Dick Tracy-esque use of color. At the time, it felt a little odd, but looking back, I realize that it contributed to a general feeling of ease. Part of me took the colors to mean that I wasn't watching anything too serious. This state of mind was necessary for the film's finale to have a more jarring effect.
This was a brilliant idea, in my opinion. In the movie, all of the characters feel a deep level us unrest, unease, and discomfort due to the incredible heat wave. Because Spike cannot make the audience feel this heat, he utilized bright colors and distracting compositions to suggest the same feeling of unease within the audience.
This contributed to the build-up to the inevitable riot at the end of the film, (at which point, the sun has gone down, so has the heat, as well as the bright colors.)