He is the brawn of the pair. Most of the time, he appears to be the dumb one when compared to Timon. However, there are a few occasions when Pumbaa comes across as quite a genius.
On another note, Pumbaa has an interesting, er, condition to contend with. Pumbaa is the very first Disney chatracter to pass gas on screen, or even at all. I wouldn't mention this if it wasn't a big deal in the movie. It was.
This was the factor that got him shunned from society. No one wanted to deal with that. Very understandable.
Those who got over it found an extremely loyal friend for life. His heart was definatly in the right place, and even though he is lazy, he can still outrun Nala.