The thing I dislike about swarming Pokemon is that unless you're lucky you are probably going to lose more time trying to finding one of them than you would gain by using them over something else that is easier to find. I think Qwilfish and Remoraid have it particularly bad because it is flat-out impossible to find one before Kanto if they aren't swarming - at least the others have that default 1% chance, and Marill and Yanma can be made easier to encounter with the Repel Trick (Dunsparce could too in theory but I don't think that Repels are obtainable at that point, and Snubbull is the lowest level Pokemon on Route 38 so it doesn't work). Maybe if there is some way to trigger swarms similar to getting evolution stones easily in Crystal...
Dunsparce looks like it would be decent if you can get it quickly. People still refer to stats frequently in this thread when judging stuff in this game, but as far as I'm concerned the minimum requirements for a stat to be considered "good enough" are lower in this game than in later generations and I think Dunsparce passes considering how early it's obtained. Obviously its Speed will be a problem but it should be able to take hits well enough and 70 Attack is sufficient for routes. Its movepool isn't too great for a Normal-type (level-up movepool is awful) but it still gets the usual Normal TMs and it can use Dig to hit everything that resists its STAB.
Yanma would be bottom tier even if it wasn't a swarming Pokemon so not much to say there.
Snubbull (GS) is virtually identical to Teddiursa, just with Sludge Bomb over Dig/Earthquake. Overall it would be a great Pokemon if you could catch it quickly since it learns a ton of moves and will contribute on routes and in several gyms. Speed will be its only drawback but even then it'll likely have level advantage and stat experience after grinding to help it out there. Also note that Snubbull has a flee chance every turn (10% chance) which is especially problematic if it isn't swarming at the time - nothing like spending ages looking for it only for it to run away before you catch it!
Marill (GS) has good bulk but it's slow and its attacking stats aren't the greatest - should be adequate for routes at least. Having access to STAB Surf and Ice Punch gives it some power I guess. Might be usable on its own merits which is what it should be tiered on, although it's hard to justify using over the numerous other Water-types that are superior to it.
Qwilfish is potentially obtained early but doesn't really have the greatest early-game movepool (I guess it has access to Swift and Headbutt for decent power in the meantime?). I think STAB Sludge Bomb from a solid Attack is pretty good though and its Speed is good enough, and of course it can Surf. It has a good match-up against Chuck (ideally you'd have Sludge Bomb already since it can't do too much to Poliwrath otherwise) and can probably contribute in other parts. So it seems like a useful Pokemon...if you can get it early. I really think it needs to be caught ASAP to be useful, but you have no control over whether you have access to it or not.
Remoraid looks amazing on paper in terms of battling, aside from its Speed after evolving. It doesn't learn too much from TM/HMs aside from Surf but its level-up move is fantastic so it doesn't matter. No doubt it can contribute in match-ups. The main issue I have with Remoraid is how late it comes. Correct me if I'm wrong but I'm fairly certain you can't go east of Mahogany Town until after you obtain 7 badges, although you can go there before taking on Team Rocket on Goldenrod. What's the strategy here? You're supposed to just rock up after beating Pryce and just hope that it starts swarming? Remember that Remoraid won't be seeing much action in Johto regardless so if you decide to just continue with the game and wait for the phone call there's a good chance you won't have it before the become champion, and that leaves you with just Kanto, most of which is stupidly easy. And I don't like the idea of just walking around doing nothing until it starts swarming just so you can use it in the few Johto battles it has access to. It'll be under-leveled too, since it's level 20 at best when everything else you're using is easily 30+. As good as Octillery may be, I can hardly call it efficient.
Naming tiers for some of these is tricky but I really think accessibility needs to be taken into account. Normally we're fairly lenient in cases where the Pokemon is kinda hard to obtain but not really (think 5% encounter rate or something) but swarming is a whole other thing all together. The risk with ignoring swarm RNG (especially for Qwilfish/Remoraid) is that you end up ranking them too high and then people planning a run through of the game look at the list, think that Snubbull or whoever is worth using, and then end up wasting time looking for it (or waiting for the swarm that comes too late and then being forced to stop and grind to let it catch up to the rest of the team) when there are several Pokemon ranked below them that would have saved them that time they lost, even if they aren't necessarily as good at battling. I would honestly not put any of them higher than D-tier, with the shittier ones like Yanma going to E-tier. It's a shame because some of them are actually pretty good when there obtained - emphasis on "when", which is the problem.