I have to write a paragraph arguing why Jordan Baker is the most important character in The Great Gatsby. Any suggestions on what makes her?
I know she's probably the least important major character, but she's the one I was assigned
Your assessment of Jordan Baker as "probably the least important major character" seems accurate, but there are a number of points available to you to make an arugment that she is indeed a highly important character.
Baker is naturally talented as an athlete. Her beauty is not "put on" and not faked. This stands in contrast to many of the people in The Great Gatsby and to much of the "built up" atmosphere that surrounds them. Perhaps this is what lends a certain dignity to her character, even if she may be more than a little petty.
The fame that Jordan Baker enjoys is a fame that she has earned though actual and public work. She has no secrets (she was caught cheating at a golf tournament and everyone found out). Her fame is not mere glamour stemming from mystery, misunderstanding or wealth as is the case with both Gatsby's and Daisy's public personas. This does not make her honest, exactly, but it does make Jordan Baker into a symbol of honesty and contrasts her to many of the other characters. (What she meant to keep private and secret, blew up in her face. Jordan is the only one who has learned this lesson.)
Finally, Jordan Baker's interest in Nick seems to be without ulterior motive. Nick is not rich, not famous, and not even particularly ambitious. Nick may have superficial reasons in falling for Jordan, but the reverse cannot be true. Jordan has an honest affection for Nick (even if it turns out to be a bit shallow).