-Decent bulk at 95/73/90. It isn't the best, but it's still decent.
-Very diverse movepool to give it a variety of options, including Wish Support, Life Orb Sets, and Hazards/Utility sets.
-Has access to the wonderful ability of Magic Guard. This allows Clefable to switch in to many different scenarios and avoid taking damage.
- Access to many support and Healing Moves such as Wish, Soft-Boiled, Thunder Wave, Stealth Rock and Knock Off.
- If running Life Orb, Clefable has Calm Mind and a wide variety of attacking moves at its disposition such as Flamethrower and Moonblast.
- Fairy Typing gives it offensive presence against common threats such as Urshifu, Dragapault and Kommo-o.
- While its defensive stats aren't horrible, they still leave something to be desired.
- Even with its good Fairy typing, it still has major threats to be worried about in Excadrill and foes and common taunt users such as Mandibuzz and the occasional Corviknight.
- It has a slight 4 moveslot Syndrome. With all the utility moves it gets, it can only get 4, which lessens its capabilities. You have to compromise.
- Its Speed again leaves something to be desired.
-Great Defensive Tank at 140 base defense. However, it only has 65 HP, but it isn't something to complain about.
- Sturdy allows Skarmory to guarantee to get of Stealth Rocks or Spikes.
- Prides itself as a great Stealth Rock/Spikes Setter depending on your team 's needs.
- Access to Roost which is good. Whirlwind is also there to annoy your opponent big time which is very useful.
- Steel/Flying typing gives it many resistances and 2 Immunities.
- Physical bulk help with Sweepers such as Physical Dragapault, Urshifu and Excadrill.
-It's Special Defense means you better switch out against Special Sweepers or you are gone.
-It's Typing leaves many key weaknesses in common coverage moves such as Flamethrower, Thunderbolt, etc.
The clear winner in.my opinion is Clefable. Clefable gets more utility and overall has more use in Switching in and providing support. Its typing provides a resistance to many OU counters which Skarmory lacks an answer to, especially the Special ones. Furthermore, Clefable can easily handle many stallers with Magic Guard. Skarmory still has to worry about Stealth Rock and Pesky burns. Even with its Physical Bulk, I still believe Clefable is better. Especially now with Heavy-Duty Boots.
Hope this helps!
Note: This is only if you want to use Clefable as a Utility Stealth Rock setter. If you wish to use it differently, there is no use comparing them like the comments stated.