Well... what do you mean by "canonical?" The "Avengers" movie is not canonical within the Marvel comic-book universe.
Also, this idea of laziness is strange to me when talking about writing fiction. Unless the story is plagiarized (which is lazy), taking someone else's character and writing a new story about him/her is arguably much more difficult than totally making up your own character. When you make your own character up, whatever you say goes. If Uberman is my character and I say he would do X, then he does X. No one can gainsay me. But if I write a story about Batman, there is an established background, character disposition, and set of supporting characters. I can't write (good) Batman fan-fic by completely ignoring those. Therefore, I am constrained much more strongly by what the "canon" elements say, than I would be if I just wrote my own story.
Indeed... one of the dreams I harbored in my youth, heck all the way into grad school, was to somehow, in some way, write my own comic-books. Now, what I really wanted to do, was to do my own characters. I had a bunch of them. I had plotted out stories for a team called the "Champions of Good" (you laugh, but I was like 11 years old, so cut me some slack) out to like "issue 75". They weren't detailed plots, just 1-2 sentence deals like, "So-and-so returns and seeks to obtain the rod of powerful evil and the Champs fight him." But still. THAT is what I wanted to write. That comic.
I knew, however, that you couldn't do that. Back in those days, an editor, I think it was Denny O'Neil (but I could be wrong) explained in a letter column that the way to break into comics was NOT to do your own creations. He said no one would take a risk on publishing new, non-established characters from a new writer right off the bat. You had to establish yourself first. You had to write one-shot stories about existing characters like Batman or Supergirl, and send them to the editor of that comic. If he liked your work, he would keep it on file and, one day when the main writer couldn't meet a deadline, maybe he'd use it. Submit your work to many editors, and one day, you might get a break.
I thought about it. Even tried to plot some out. I couldn't do it, and I eventually gave up. Know why? It was too hard to use an established character. Although I liked reading about Batman or Supergirl or the X-Men, I found it too hard to write someone else's characters. I had a better feel for my own.
So, I don't think it's lazy to write fan-fic. If anything, having written (but never published) three full fantasy novels and parts of two more (each of them hundreds of manuscript pages long) but not ever been able to turn in a single one-issue comic script about an existing character, I say the lazier way is to just make up all my own stuff. Because then I can do whatever I want, and i don't have an existing continuity or established character to worry about.
So... if you wanna rag on fan-fic for some other reason, go ahead. But don't call it lazy, because it's not.
Again, I'm not talking about plagiarism (copying whole stories, etc).