I suggest you try it and see if the effect improves things for you. If Metamucil give you gas, some people prefer plain psyllium, or Citrucel, or Benefiber. I've been happy with Metamucil for years.

I prefer to reserve the word "diarrhea" for watery stools with urgency. If there's no urgency the consistency of the stool is much less important, and most J-pouchers are best off with a soft stool consistency, a bit like pudding in thickness. That minimizes any straining to poop.

I never needed Metamucil, etc. but when I tried that sort of stuff in the beginning, because they told me to, it just didn't work for me.  Give Citrucel a whirl, maybe, to thicken your stools, some people like it better (I, too, don't say "diarrhea;" better to say "loose stools" if not ill/with urgency... I get picky about that, being a nurse). 

I actually prefer looser stools... Easier to get out.  I'm continent whether liquid or other, though, and rarely get irritated/butt burn.