I’d argue the list could be:Top 5 Worst Types in SS Monotype
5th place: Bug
The addition of Heavy-Duty Boots means Sticky Web teams are much less consistent now. Bug also lost its main powerhouses in Mega Pinsir and Mega Heracross, so Bug teams don't dish out that much damage. The type itself is bad offensively and defensively, they're stuck low speed and mediocre bulk. While Bug has no problem setting hazards, getting rid of them can be a big problem: your options are more or less limited to Armaldo. It's also just straight up outclassed: Fire, Water and Electric all do the all-out offensive type better. While you could do worse than Bug (see below) you could also do a lot better.
4th place: Fighting
If you're laddering with a Fighting team, get ready to be stopped cold by Toxapex, swept easily by Psychic and Fairy, and dread seeing Flying (arguably the best type) in the team preview. You also need to get ready for never having enough slots on your team, and thus deciding what you want to lose to. Bring Toxacroak to check Rain HO Water, but then you'll struggle with balance Poison. Bring Scarf Buzzwole or Heracross to check Psychic, but then you'll struggle more with Fairy and Flying. Hazard control is another serious problem for Fighting teams. Cobalion is the only good Stealth Rock setter, and unless you want to run Rapid Spin Hitmonlee or Defog Hawlucha (you don't, Hitmonlee sucks and Hawlucha has better things to do) once your opponent gets the hazards up, they're staying there. Stealth Rock is a minor inconvenience, but God help you if they have Sticky Web since you're so reliant on Scarf users. It doesn't even reliably beat Steel, since the Aegislash + Celesteela/Corviknight/Skarmory core shuts down most Fighting types.
3rd place: Grass
When it comes to bad matchups, Grass arguably has more than any other type. Bug, Flying, Ice, Fire, Poison, Dragon, Steel, all of them eat Grass teams for breakfast without even stopping to chew. While the Grass type does have some nice resistances, it ties with Rock for the most weaknesses, meaning almost every type has Pokemon Grass types fear - Dark has Goltres, Fairy has Togekiss, Electric has Zapdos. It's also very weak offensively, being resisted by 7 types. Almost everything on a Grass team is weak to Fire, so against any team that has them, you're in trouble. Cradily offers a neutrality, but it's very passive. Grass teams in general tend to be quite passive, and unlike Poison don't have a Regenerator core to fall back on (I mean technically you could run Amoongus + Tangrowth, but why would you?).
2nd place: Rock
Now, take the lacking Speed of the Bug type, 5 common weaknesses of the Grass type, and add a general layer of suckiness on top, and you have the Rock type. Good luck finding a team of any type that has no Water, Grass, Ground, Fighting or Steel types. Even the types it should do well against like Bug, Fire and Flying all have Pokemon that can sweep Rock teams: Swords Dance Scizor, Bulk Up Cinderace and offensive Landorus-T, respectively. Hell even Normal can bring Bewear which gives Rock a lot of trouble. Hazard removal's a pain too, unless you want to bring Armaldo or Defog Aerodactyl. The only type Rock can reliably beat is Ice, and even then, it still doesn't like dealing with the likes of Galarian Darmanitan.
1st place: Normal
What could be worse than a slow type with lots of weaknesses, low speed, bad hazard control and very few good matchups? A slow type with no resistances, bad hazard control, a small pool of viable Pokemon, a propensity to get worn down quickly and not one reliable match up. Ghost is the closest it has, and even then Ghost teams can wear Normal teams down quickly with entry hazards and status. The Chansey + Porygon2 walling core is nowhere near as good as it might appear on paper, as both are reliant on Eviolite and thus have to be very careful of Knock Off and are very susceptible to passive damage. It also doesn't have any good sweepers apart from Snorlax, which isn't too difficult to stop if you don't let it get multiple Curses off. Gen 8 just gutted Normal - it lost its main hazard setter in Smeargle, its main sweeper with the loss of Z Moves, its anti-offense option with Mega Lopunny and its Fighting check with Mega Pidgeot.
Grass, I believe, is the best of the worst. You could argue for Fighting to be above it, but I put it at a close second. Grass has a solid defensive core to support the offense and utility it has. Rillaboom hitting hard with its STAB and coverage, Whimsicott disrupting setup sweepers and walls, Rotom-Mow either being speed control or a status inflicter, Zarude being speed control or a setup sweeper, same with Celebi.
Fighting, on paper, has great mons. Gapdos, Urshifu-S, Hawlucha, Terrakion, etc. But like you said, it has 6 slot syndrome. Also, without a reliable core, it’s tough to support these heavy hitters efficiently which is why I put it below Grass. On the flip side, I put it above the other 3 because it has arguably the best mons out of these 5 types.
Bug has been terrible since gen 7 and it only got worse in Gen 8. The webs are still good because while HDB exists, not everything runs them so webs still achieves the same utility for Bug. In addition to HDB’s existence, most Bug teams opt out of hazard removal. Putting boots on the ones that need it (Volcarona, Araquanid, Ribombee) and using other items on the ones that don’t need boots (Scizor, Heracross, Buzzwole). I see Bug as a worse Fighting offensive-wise.
Normal is thereciprocal of Fighting. Great defensive core, bad offensive core. Smeargle was HO Normal’s main hazard setter, balance Normal’s main hazard setter was Chansey. The defensive core is still good, swapping Eviolite Chansey for HDB Blissey. It’s just not able to crank out as much immediate damage outside of Diggersby (maybe Indeedee too). It still has good matchups against decent types like Dragon and Dark. Just not as oppressive as it used to be when it had megas and Staraptor.
Rock is BY FAR the worst type. The loss of megas probably hurt Normal the most proportionally, but the pain Rock feels from losing its megas are almost up there. Losing Rockium Z hurt a lot too because now it’s harder for Stakataka to snowball. The offense is okay, the speed is whatever without webs, defensive core is severely lacking. You are setting Monotype to the highest difficulty setting by playing Rock.