To make dragon a little less overpowered, they needed to make it weak to another type.
No they didn't. There were a number of better ideas that could have been done to weaken the Dragons that already exist and that includes making Ice a more viable battle type rather than leaving it as a useless brittle type that people would rather sacrifice the STAB they could have in favor for more type coverage.
Fairy Type was a horrible addition that only brings 1 positive to the competitive field and that's an increase to Poison type's powers. Though this added strength means absolutely nothing as the Pokemon are still pathetically weak.
Dark and Fighting didn't need more weaknesses. Bug didn't need to be resisted by another type, and Dragon certainly didn't need another weakness or immunity against them. Steel should still resist Ghost and Dark since it still plays with the mythos that Fairy Types carry since Steel/Iron/Metal has always been a sign of holy weapon against creatures of magic and shadow.
All Gamefreak needed to do was give better move coverage to older Pokemon, changed the stats as they did do in X and Y but to a higher level to make the weaker ones that were in need of a competitive boost a viable power against the Pokemon that were on the top.
That brings in to the overall complaint. People keep *****ing about how stale and drab the battles were, but then they'd use all the same tactics that everyone else used, wouldn't even try to come up with more counters to try and change the teams, or do anything worth merit other than open their mouths and complain about why they can't win because everyone used the same things, no one had an original thought, or they were just to lazy to do any work to try and change the battle scene.
They could have weakened the power of Dragon attacks, given Ice a defensive boost and given them resistance to Dragon, as well as redistribute the high end Dragon's stats to give them less effective damage, though that seems a bit out of the way as they are supposed to be powerful.
Though even on official tournaments, the high placing players barely even used dragon types on their teams with an exception to 3 pokemon in general as all I usually saw repeating was Garchomp and the Lati-twins.
You'd have occasional dragons show up, same reoccuring ones, over and over in the lower ranks, but the higher teams had more team strategy than simply relying on a Dragon type to push them through.
Fairies are pretty much on the same level as Poison and Ice stat wise. They aren't much better than the two, are both just as brittle and easy to take out, even with resisted defenses, unless they are Carbink and Klefki who both seem to be illegitimate children of Shuckle with better HP.
Best attackers (Non-Legend) seem to be Togekiss and Azumarill. Gardevoir is nice but needs her Mega for some battles otherwise she's about as frail as she was in the past with a common weakness to Steel, a comical weakness to a type she's been strong against for 4 generations, and keeps her Ghost Weakness, while now taking neutral damage from Bug and Dark, but can still take severe damage from a Dark type attack.
They were an unneeded change to the game, but not an unwanted one. They could have done so much more without the need to add another weakness, though that still is not going to stop Dragon Types since they can just swipe a lot of fairies to the side with altered moveset to include Steel and or Poison attacks. Some can do just as much damage with Earthquake alone and wouldn't even need Steel/Poison attacks.
Dragon's weren't overpowered, just overused. Their prowess is grossly exaggerated otherwise Lance and Drake would have been impossible to beat even in game without the use of Dragon Types yourself.
/rant