Trubbish is not garbage, but it's certainly not a fantastic, A-rank Pokemon. It has a notable niche of absorbing Knock Off, and being able to set down hazards, which are both very nice. However, it is not only walled by Misdreavus, but by every ghost with the exception of Pumpkaboo and Phantump, and virtually every poison type. Yes, that includes Gastly. In addition, Trubbish is prone to being "revenge killed" by abra, and, if it gets a free switch in, Diglett can trap and OHKO it. Drilbur, as well, can both OHKO with Earthquake and also spin away any hazards Trubbish sets up. Trubbish is absolutely a good answer to Pawniard, and has its niche, but has flaws that ultimately prevent it from A rank.
Whoa whoa whoa, don't hate. Trubbish is still an amazing poke in the current meta. Meditite, practically the only Fighter that can beat it, is banned, and Krow is one of the few physical attackers that had a chance to OHKO the tresh bag (thats right, it didn't even guaranteed an OHKO on something that didn't run Eviolite.)
So your main argument is that quite the number of pokes wall it/easily kill it. You put down 4 main types: Ghost, Ground, Psychic, and Poison. However, when you're only effectively beaten by 4 out of the 17 (iirc) types in the game, along with
beating 3 of the most threatening types: Fighting, Dark, and Fairy, you have quite the amazing typing/good movepool. Of course it's going to lose to Psychic types and Ground types, just like Mienfoo loses to Fairy types and Flying types, that's not the point. It can do something virtually no other pokemon can do: shut down Mienfoo, Timburr, Croagunk, Pawniard, Houndour, Spritzee, and Snubbull
AND set up Spikes extremely reliably. That is an amazing niche, more that enough to bring it to A.
However, like every poke, Trubbish does have its problems. It can't beat the aforementioned Ghosts, Psychics, Grounds, and Poisons. Now, let's see how many pokes from each group can actually beat it.
Ghosts: Misdreavus, Gastly
Psychics: Abra, Wynaut
Grounds: Drilbur, Diglett
Poisons: Foongus, Croagunk, Trubbish
...
That's it?????? 9 amazing viable pokemon to take on Trubbish? Or can we go further. I want to look particularly at the Poison group and see their 1v1 match-ups vs Trubbish.
Foongus: Stall War. Both have reliable recovery. I don't have access to the Calc, so I'm going to assume it doesn't 2HKO (which it very well might). Anyways, Trubbish outspeeds and can easily set up Spikes. Foongus spores and it proceeds to hit Trubbish with tickling moves. Once Trubbish wakes up, it can set down the two other layers of Spikes. Foongus is Trubbish's Spike fodder, letting it get at least one layer down easily, all three if it stays in. Foongus can only hit it with pathetically weak moves. We haven't even mentioned how much damage Gunk Shot does to Foongus ( I would think 2HKO, but whatever). Not something I would call walling.
Croagunk: Again, Spike Fodder. However, Trubbish easily wins this match-up 1v1 if it really wants to. Croagunk can't touch it. At all. Though Trubbish does poor damage as well, Croagunk doesn't have reliable recovery meaning it easily winns this match-up. It can even set-up Spikes in Croa's face if it wants to. Again, doesn't wall.
Trubbish: Spike spamming, yay.
So basically all the Poisons are knocked out, but it still does its amaZing niche effectively, and with 6 real answers .-.