I asked around on Smogon Gen 9 OU chat and I basically got what Mangy Vargant said. It depends.
Normal poison is really good against opponents you expect to switch out a lot, such as Cyclizar and Zapados, so use Sludge Bomb there for a higher chance for normal poison there.
Severe poison is good against Pokémon you expect to be staying in for awhile, such as stallers.
In your case, I would suggest Sludge Bomb since your Munkidori seems to be more offensive and could use the more immediate damage that comes from regular poison, and there’s a 30% chance for it instead of 10% with Sludge Wave. For a bonus, I think Choice Specs or Scarf would be good with your Munkidori.