I have seen many authors use the word adorned as a synonym for dressed, usually in this manner:
He was adorned in a fine silk doublet of the deepest green, so dark it was nearly black.
This is incorrect. Adorn means to enhance the appearance of something (usually with beautiful, individual objects); it is generally paired with the preposition with, not in. For example:
The Christmas tree was adorned with lights and tinsel.
The king demanded that a tapestry depicting his victory be woven to adorn his throne room.
Her paintings adorned the walls of the manor.
The subtle difference here is that adorned implies that an object has been hung or pinned upon something; it is an individual embellishment or series of embellishments. Clothing (and by extension, dressed) is more encompassing—a complete covering instead of a specific embellishment. Adorned is the equivalent of painting your address on the front of your house in fancy calligraphy; dressed is the equivalent of giving the entire house a new coat of paint.
(It is fair to note that while adorned cannot be used to mean dressed, dressed can sometimes be used to mean something close to adorned. This is probably part of what leads to confusion between the two words.)
Usually, this inappropriate use of adorned appears when a writer wants to say that a character was dressed in something fancy, but wants a more impressive word than dressed. Clothed is your best option, or you could say that the character wore or donned the clothing. But don’t let yourself fall into the trap of abandoning the most effective, useful word for what you want to say simply because it seems common! Common is good—it’s understandable, unpretentious, and undistracting. Save your searches for fancy synonyms for something more important than dressed.