Rotosect
Banned deucer.
Using single, extreme examples (seriously stop using the "Regigigas proves BST doesn't mean anything" argument because we're not idiots and we all know what's the deal about it) to support your counter-argument isn't a good idea.A couple of things wrong with this.
First, a pokemon's BST doesn't determine it's status, otherwise Breloom in BW would be NU and Regigigas would be Ubers.
Second, a pokemon's Tiering last gen has zero standing on its Tiering this gen. Gastrodon was NU in gen4 and OU in gen 5.
Third, you might be underestimating just how powerful that fairy addition is. Azumarill getting it was enough to bump it up to OU, as it gave it secondary STAB and, more importantly in relation to Clefable, more resistances. Clefable's only weakness got turned into a resistance in exchange for two relatively uncommon weaknesses, making it the best unaware user IMO.
Aegislash exists, pretty much. Being weak to dark and ghost really hamper Jirachi now.
If you take a look at the average BST, you will clearly see that it gets higher as you move up in tiers, so there is an obvious correlation bewteen BST and tier placement, just like there is a correlation between tier placement and viability. I know correlation doesn't necessarily equal causation, but we all can put two and two together and see how competitive Pokemon works in the big picture.
Been said that, the problem with Clefable is that it just can't stand up against powerful neutral hits, which are everywhere in the tier.
Charizard, Keldeo, Aegislash, Landorus and many other offensive threats can comfortably 2HKO it even with neutral hits so Clefable has a hard time switching and setting up.
Also "best Unaware user" doesn't mean much when its competition is Quagsire, Bibarel and Swoobat.
It's a solid pokemon but A+ is too much for it.