Being fashionable is adverbial periphrasis of mode, which means that anything material, music, custom, or attitudinal is carried out or repeated by a large social group. It applies to many things and many acts of the society. By repeated regularly among many individuals. You are the conclusions of its etymology: gallicism mode which in turn is a Latin cultismo, modus, which means "mode and measurement", which, by extension applies to any mode of dress, Act, listening to a type of music, see a movie, etc.
