No, it isn’t. Writing isn’t anything in particular to different authors, or to a single author writing for different purposes and different audiences on different occasions.
When Ernest Hemingway (or Red Smith, or Paul Gallico) sat down at his typewriter and opened a vein, was it difficult for him? Easy but painful? Gratifying? Or all four?
Writing isn’t a single skill, so even if one part comes easily – the conflict of a plot, a provocative character, or unexplored setting – the next can require time and effort, blood and sweat, to get the words right (as Hemingway said, for sure this time, to the Paris Review).
Writing fiction is a peculiar activity by any account. You sit down in a room alone, ignore the actual world around you, and inhabit another instead of your own devising. This imagined world may be far worse than the actual world, crueler or more frightening, but a writer chooses to linger there, constructing it image by image, sound by sound, word by word.
To do this requires some generative ability to dream up new people, conflicts, and reversals, and some critical skill to select and shape those ideas. A writer with ready access to his or her imagination may find the first task easy enough but struggle over the second; or a writer well versed in craft may have no trouble revising and editing but a hell of a time unearthing new ideas.
Dialogue comes easily? How about description? Got a big vocabulary? How easily do your sentences flow, phrase after phrase? You might have your basic grammar down, but do you go with the proper word here or the way most people speak? And is that expression current slang or the sound of your generation?Add to the mix the importance of observing human behavior and empathizing with feelings, and you have an idea of the range of abilities with which a writer will have more or less facility.
Then there's the difficulty of telling the truth, and the desire to see it in print. It can be a snap to fill a page with something that needs to be said -- or excruciating to write one word in lipstick on a mirror, or in dust on the trunk of a car.