Steel is one of the most prominent types in the game and most Fairy types don't have the bulk to take strong neutral attacks or have additional weaknesses because of a secondary type. Excluding Xerneas, the only viable pure Fairy is Clefable. Having to prepare for the possibility of fairies being on a team doesn't make them broken. You've also got to prepare for Steel, Water, Fire, Grass, Ground, Flying and to a lesser extent Dragon, Rock and Fighting.
Ground and Steel are the best types in the game. Ground has been pretty much universally recognised as the best type in the game in gens 6 and 7 and as second only to broken or near-broken types in 1-5 for several generations now and Steel is the best defensive type by miles and has been since its inception.
Few Pokemon carry Fairy moves as coverage and if they do it's usually Dazzling Gleam not the two more powerful moves. On top of that the number of Pokemon that can OHKO with Moonblast and Play Rough are not particularly high. Compare that to Earthquake which reaches the 100BP tier, hits a lot of types super-effectively and is much, much more widely spread or to Superpower and Close combat which are also quite widespread but have base 120 power and still hit a lot of types supereffectively.
Mimmikyu wouldn't even be in OU if it weren't for an exceptional ability. It's not a particularly good mon, it hasn't got a lot of staying power and it is barely relevant in OU anymore and will probably see a demotion to UUBL if not UU soon. Of the Tapus, only two regularly make use of Fairy moves at all and all of them tend to use their non-Fairy STAB more often because they hit more (crucial) types. It's not the Fairy type that makes Lele, Koko or Bulu threatening, the only pure Fairy types usable in higher tiers (excluding Xerneas) are Clefable and Sylveon and I wouldn't call either the premier mon on their tiers now. Discouraging Dragons isn't a point to Fairy being broken, it is why Fairy exists for the most part and is why Dragons now aren't broken. Magearna you have something of a case for but its hard to call because it's also part Steel, has a great stats, a killer ability and a versatile movepool.
I'd hardly say most of them got a big boost. Off the top of my head the only Pokemon that saw a really enormous boost was Azumarill with Clefable experiencing a more moderate rise. Everything else had very minimal improvement.
I'm not going to try and argue Fairy is a bad type because that's stupid. It's a very good type. It's just not so good that it breaks the game. Types that broke the game were Psychic in gen 1, it had a single weakness that might as well have not existed, was only resisted by itself and had one of the best offensive moves in the game in Psychic or Dragon in gens 3-5 where it was pretty much exclusively held by heavy hitters and only resisted by Steel, which most Dragons could cover with Fire or Ground moves. Compare this to Fairy that not only finally balanced out the type chart for the most part, but has weaknesses that are either common (Steel) or not uncommon (Poison) and few resistances/immunities... 3/4 being to types you'd have to be stupid to use against it anyway. Fairy is a great type but it's not even close to broken.
