Clefable outclasses all other fairy types, deal with it, azumaril is a 1 trick pony everyone knows its going to do bellydrum, m-diance can be countered pretty easy by landorus therian, or anyone with earthquake or water types. Though I think m-diance might be the 2nd best fairy in OU.
Shadow tag was a problem so they banned it, so why not arena trap? Oh right cause smogen logic. It really makes 0 sense to have arenatrap be allowed, but shadow tag is banned.
Clefable does NOT outclass Azumarill or Mega Diancie and affriming that shows that you don't understand this metagame very well and that you haven't analyzed Azuma and Diancie's potential yet. Clefable may be great and i think you said that it deserved to be banned before but it does not, its defensive stats aren't huge for a wall so it ends up being 2HKO'd by wallbreakers, Specs Latios with Psychic is getting more common and that set scores the 2HKO on Clefable, regardless of its spread and overall it's pressured easily, if not at 100% hp, it stops being able to switch on certain opponents. Also steel types such as Excadrill, Heatran, Mega Scizor, Jirachi, Mega Metagross, Ferrothorn, Magnezone are everywhere and even tho they aren't hard counters due to moves such as Flamethrower and Thunder Wave, they are checks regardless and there are other pokemon that check Clef, for example poison types like Nidoking and Gengar and opposing CM users such as Suicune, mainly if it has Roar to phaze it or Sub + Protect and Psyshock users such as Reuniclus and Slowbro/King. Obviously Clefable is great and has a ton of positive aspects but i'm just focusing on the negative ones and on its checks so you can see that Clef is not unbeatable, also Clef not being a physical water type that has Huge Power and being able to utilise a Choice Band or a Belly Drum set effectively should tell you that it's not outclassed by Azumarill and don't compare Diancie with Clefable either, Clefable is a bulky wall, Mega Diancie is a fast Rock-type wallbreaker so they clearly do different things.
You also said something about Landorus-T earlier and from your post, it seems that you also wanted Smogon to ban Lando and i do not know where are you coming from, i often read on showdown things such as "omg they need to ban lando, it's too broken" and i don't understand, sure that it's S rank and is an incredibly splashable pokemon but it's actually
healthy to the metagame, it checks threats that would otherwise be much more dangerous such as Excadrill, Mega Lopunny, Terrakion and Mega Charizard X. Lando-T is not the pokemon that you see on the opposing team and think "omg lando, i'm going to lose" mainly because it is way too common not for you to cover and it's easy to check it, especially when the own Landorus-T checks other Landorus-T but it usually doesn't hurt including pokemon such as Tangrowth and Rotom-W on the team since they are very good and also check Lando but other common threats such as Keldeo, Slowbro, Starmie and Kyurem also beat it. The most common Landorus-T sets you will see are Defensive and Choice Scarf, the defensive set isn't so powerful as it's not an offensive set, easily worn down since it tries to check multiple pokemon and then lacks recovery not to mention that many pokemon have no trouble fitting Ice type coverage moves on their sets for example, every Mega Medicham carries Ice Punch in this meta and Lopunny can use Ice Punch or even Ice Beam on the set without losing much. The Choice Scarf set may be good but it's not overpowered either, it is a U-turn spammer so it invites Rocky Helmet users such as defensive Landorus-T and Tangrowth to come in to wall and deal chip damage and it cannot simply freely use alot of EQs either because ground immunities are everywhere. Also, i fail to see how Landorus outclasses all ground types when Garchomp, Excadrill, Dugtrio, Diggersby, Hippowdon, Gliscor, Mamoswine, Nidoking, Gastrodon and Seismitoad are all viable in this meta...
Also, Dugtrio may be awesome and i understand where are you coming from when you say that it's unhealthy and frustrating to play against, getting rid of Heatran or Diancie can win you the game but Dugtrio can't trap "the entire meta" as you say, its base atk is pretty low, only traps grounded pokemon and its fraility equals over reliance on Focus Sash or predictions and if you try out Dugtrio, you will notice that it's total deadweight against some teams pretty much and its effectiveness on stall (and the effectiveness of stall in general) has been reduced due to the Sableye ban. By the way, not being able to switch in safely on Heatran (the mon that it likes to trap the most) due to the possibility of taking a Lava Plume and being burned or in the worst case, taking a Magma Storm which kills it through sash is an issue too.