I'm not sure where you're getting the "average-looking, girl-next-door" vibe.
I find the vibe is everywhere in the books. Both in the way Katniss thinks of herself and in the comments from some of the other characters. How a person carries himself/herself has a large bearing on how attractive that person is to others. Some of the things Katniss says in the first book are that she "never smiles except in the woods", "just keeps to myself", she "has a lot of practice wiping my face of emotions". I'm not sure about the exactly wording, but she thought the reason why she was never popular at school was because she was just an unfriendly person.
During the training session with Haymitch, the general vibe was that she "just doesn't have the arrogance. Too vulnerable for ferocity. She's not witty, sexy or mysterious". Haymitch himself thought she appears sullen and even hostile, and has as much charm as a "dead slug". That she is "no one at all".
Finnick had a casual dig at Katniss by asking how she could stand her own image in the mirror. Even though Finnick might have seen plenty of women in his times, and in his unpretty state he may still be pretty hot, I wouldn't think he would say that to someone who actually looks gorgeous.
All these would not give a vibe of her being an attractive person in a romantic or even in a social way. She could be one of those people who would not entice you to open her up, but once you do open her up she can be as endearing as anyone you'll ever meet.
I wouldn't take much out of Darius' flirt on Katniss when he told her that she ought to buy one of his kisses. It's his egotistic boast than his desire for her. Katniss thinks he is boyish for a peacekeeper, implying a degree of immaturity.
The other person who finds her attracive is Peeta, but that was a crush from childhood. Obviously there are other qualities in her that attract Peeta and others, but not in a romantic way. (In Peeta's case, these qualities attracted him romantically.) I see that the effect Peeta talked about refers to the "fire" in her. She said she will not go down without a fight. She would not show weakness by shedding tears. She is very confident about her physical abilities. The bow and arrow is her life (she said "goodnight" to it in MJ before she headed for the nightlock). She knows she is quick and agile, and despite being thin she knows she is strong. Once she has the bow in her hand in the first games she became fearless like the Careers. When she's not conscious of it I'd say there is an air about her that would send a message to the oppressed (hope, solidarity) and the oppressor (threat). What message does she send to the Capitol crowds? Maybe a message they have never seen before and stoke a hunger (how rare!) inside them? Maybe for change? No wonder Snow finds her so dangerous.
Once she dons on Cinna's dress, she said that "I am not pretty. I am not beautiful. I am as radiant as the sun." The radiance of the sun is the effect that she has on people. Not her beauty or charm.
This gives a good description of what she is in the eyes of the public and what she looks like with makeup and clothes her prep team puts on her and when they film her.
It's what the propaganda team like the public to see her as. Why the need for so much make ups and photoshopping? Katniss could not relate to the image she was seeing - tall, imposing, sexy. The defiance in her eyes, even if they were there at the beginning, may have toned down somewhat after all the anxieties and battle weariness.
