In my view, it would undermine what Pumbaa's backstory tried to tell us in the original film during the song, "Hakuna Matata". Plus, Pumbaa's backstory already briefly tells us his backstory in the dialogue when he first meets Timon in the movie:
Timon: Hey, what about you? You're all alone. You are all alone, aren't you?
Pumbaa: Yeah. Most animals give me a pretty wide berth.
Timon: They do? Even the hungry ones?
Pumbaa: Yeah, I drive almost everyone away. People see me comin' and they run for cover. They can't get out of the way fast enough.
Timon: You don't say, you don't say. Must be the gleam on those vicious tusks, huh? Strikes fear inte their hearts.
Pumbaa: Actually, they say I wreck their appetite.
How much further can you get than this to get what Pumbaa's backstory is like?
Also, I see
TLK 1½ is more from Timon's viewpoint of the original film than that of Pumbaa's, so it was best to tell his "official" backstory.