I think Farley might have taken the character too far over the top and the movie might not have been quite as good because of it.
Interestingly enough, it seemed to have been the opposite. The people involved were talking about how many of the readings were really heartfelt. There's a scene where Shrek is singing, it was intended to be funny, but everyone involved said Farley made it touching instead, and it worked. I would have assumed that Farley would have made Shrek full-throttle, but it seems that wasn't the case.
Interesting sidenote is that when Shrek does "finger quotes", that was based on Farley in real life.
This was all from a Playboy article in which they talked to a bunch of his closest friends, family and associates about what happened in his last days, sad stuff.