Intentional mash-ups can be fun. The heroine runs away in a pseudo-medieval fairy tale world and buys a hot dog from the hot dog stand, or the princess in that world munches on popcorn for a tournament where her beloved crab husband turned into a man to joust. But it's definitely comic and requires self-awareness.
It's possible to rip off ideas from various eras and fit them together to make a strong and believable story, but that takes even more reading than sticking to one era, so you know what fits. A society with a strong social hierarchy is not going to have a modern day support group in which the cursed can gather with everyone from nobles to stable boys being equals in it. It could be nobles being Lord and Lady Bountiful to those who share the misfortune of being cursed. It might even be a society of some kind from the 19th century, though that would probably still be aware of the social differences even as they work together. It could be a religious confraternity from the medieval era that was specifically organized for the pious purpose of mutual support under the patronage of, oh, St. Benedict, where they might make a conscious effort to remember equality before God.
The big problem with this is not generally throwing in something and not caring whether it fits. It generally is realizing that what you automatically think of -- because it's current -- is not the necessary, or even necessarily the normal, way of doing things. Research is great as long as you have enough awareness to realize that you have to do it. Many writers sail merrily along with the modern elements without even realizing it.