Are there legal ramifications for passing fiction off as nonfiction?

Depending on the subject matter, yes there can be serious ramifications on passing fiction off as non-fiction. You can lose your job, cause people to make dangerous and even deadly decisions, stir up controversy based on falsehoods or innuendos, cause unnecessary hysteria, et., etc., etc. Cause in point: a number of new parents refused to have their young children innoculated because they were told that a study showed that mercury in the shots caused autism. That report was false. But a huge number of children have been put at risk for avoidable and in some cases deadly illnesses because a piece of fiction was passed off as non-fiction.

It depends on many factors, but the legal repercussions are not necessarily severe unless the story can be considered defamatory to a person, in which case there would be potentially costly effects. The courts, for example, have held that even if a movie or book uses the phrase "based on a true story", it is not actually a guarantee or statement on its authenticity.

No and the question is dishonest.

Passing fiction off as nonfiction has caused terrible trouble for the author and the publisher, especially in the past few years. People have lost their jobs, their contracts, books have been recalled, Oprah has gotten angry. These books are known as literary forgeries. If the author has a publishing contract, they might be subject to whatever remedies the publishing house decides to pursue. If a journalist fakes a story or magazine article, as Stephen Glass did, he or she will usually be considered a former journalist or soon to be barista.