ABSOLUTELY NOT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!­!

Absolutely

DUHHHH OF COURSE NOT!

I really wouldn't say she is at all!

Bella has no exploitable flaws, there, I said it.
"She's clumsy!" Naysayers will mention.
Everyone is, to some degree. Bella is shown as bad at sports and incredibly clumsy. Same as myself but I don't go "Oh, clumsy me!" every time I trip over. Clumsiness is just not that much a flaw.

Now, Mary Sues are amorous things aren't they? Mary-Sue will have many boys in love with her from the moment she steps in a room, all the girls will be jealous of her and she could very well sleep with the whole town, but she won't because she's not that girl.
Bella fits in with that exact description.
Mike, Eric, Tyler and Quil are all mentioned as having a crush on Bella, not to mention Bella's relationships with Jacob and Edward.
For someone that classes themselves as plain, that's good going, I'd say.
Now, Edward, of course, is described as gorgeous. Personally, I'd like a guy to have, well, a heartbeat, but hey, whatever. So how is it, day one, he is attracted to her, when she insists she's awkward and average?
Oh...right. Mary-Sues like to state their averageness and with shrug off any attempts to prove otherwise.
Now, I don't give two sods about shipping in Twilight. The way I see it, Edward is abusive and stalkerish, and Jacob is obsessive and...stalkerish. Wow. She really knows how to choose the hot and crazy ones.

Bella is described by Stephenie Meyer as:
"very fair-skinned, with long, straight, dark brown hair and chocolate brown eyes. Her face is heart-shaped—a wide forehead with a widow's peak, large, wide-spaced eyes, prominent cheekbones, and then a thin nose and a narrow jaw with a pointed chin. Her lips are a little out of proportion, a bit too full for her jaw line. Her eyebrows are darker than her hair and more straight than they are arched. She's five foot four inches tall, slender but not at all muscular, and weighs about 115 pounds. She has stubby fingernails because she has a nervous habit of biting them."
Which, if anyone has ever checked, is exactly how Stephenie Meyer looks like.
As mentioned, plain? In my minds eye, that paints a picture of an attractive enough woman. Except, of course, Bella, at 5'4 and 115 pounds, is in fact close to underweight. She does no exercise and eats quite a lot of calorific food in the books. Sailor Moon Syndrome, anyone? Usagi in Sailor Moon eats a lot of junk and never gains weight, maybe Bella is Sailor Moon?
I mean, even Bella's eyebrows are described. Isn't that a bit much?
A small character description and that's it. Some authors, such as Terry Pratchett, rarely ever use deep character descriptions, preferring to leave it to imagination. Some authors, like JK Rowling, manage to give only a line or two of description and you get the character fully formed in your mind. We are never told Ron's eye color or what Seamus look like, but even before the movies we imagined Ron with blue eyes, or Seamus with a cheeky grin.
...Although, to be honest, Bella gets nowhere as near an amount of description time as Edward.

Bella, in Breaking Dawn transformed from a run of the mill Mary Sue, to a God Mode Sue.
She's shown to have more restraint than any other vampire because "she was expecting to become a vampire". Nice one, Ms Meyer.
She manages to pop a kid out before the ole biological clock stops ticking? Check.
More graceful than most vampires? Check.
And does she have a unique power that just so happens to be able to defeat the most powerful vampires in the world? Oh God yes.
So she gets the guy, she gets immortality, and she gets everything handed to her wrapped up in a silver bow.
Wow. Just... wow.

How could anyone legitimately choose "No"?

Not at all.