In short:
Absurdly.
In long:
They at overpowered, but not for the same reasons everyone said dragons were. Mostly, its a few choice individuals that are overpowered, namely Gardevoir.
Mostly it is due to how they are super effective on three of the most effective damage dealing types, who are all very well established to have their weaknesses, and it is easily known how to use them, the only reason many people said dragon type was overpowered was they were using the wrong choice in words, dragons were 'overpowered' (I use quotes because, they were, in reality, actually pretty frail to anyone who knows how to use them) because of their massive stats, due to most being pseudo-legendaries, or even legendaries themselves. Yes, they had only 2 weaknesses, but dragons were often the victims of a dual-weakness, and no super effective STAB except against one of their own and, with a few exceptions, relatively low defense. Where as Fairies so not have any of these problems, they have massive special attack and defensive stats, meaning they dishout a lot of punishment and take almost none, take no damage from dragontypes, have the same number of weaknesses as dragons, albeit they are far less common and viable to be used against fairies, are super effective against 3 types, and, this has to be the biggest reason, one of the types they are weak to, poison, may as well not even be there because most all fairies can learn the TM Psychic.
YEah, dragons may be overpowered because they hit hard, but even so, that is their big flaw, that is *all* they can do for the most part, even their support moves, like dragon dance, only exist to further beef up their own offensive potential, and leaves them vulnerable, where as fairies are not as vulnerable, do to their massive defenses, very uncommon weaknesses, massive movepools that can negate any cause for alarm if facing one of the weaknesses due to a lack of viable anti-fairy (steel and poison type) attacks, it all culminates in a type that is far move of a problem that the type is was supposedly meant to balance out. YEs, dragons were powerful, and we all love them because of that, but even then, they weren't infallible, quad weaknesses, and most being heavily reliant on knocking opponents out fast made them easily crippled by status moves, (most dragons are physical destroyers, a burn would more than destroy a dragon's potential) where as no such conditions exist for fairies, and many know moves that can heal themselves with draining moves, something very few, if any, dragons can do.
Its actually very unfortunate because the whole purpose of the gen 6 games was to make the fans love the idea of fairies and Mega's, and in the process, it made both far too good to be likable, leaving a lot of people split on them. I'm not going to lie, I am very fond of dragon types, I always use one, and going against (gen 6) fairy types is down right unfair for a dragon type, then generation 7 ones are far more balanced, but it seems the damage has already been done. I would love nothing more then to like fairy types, but after seeing what they are like, there seems to be no viable justification for why they are as powerful as they are.
Sorry if that was a bit long, but there is a lot to say on a topic like this.