RE: ADDITIONAL INFORMATION...
It sounds like she likes you, but is afraid. Are you brave enough to ask her straight up? The "obvioulsy fake" flirting might be because she really likes you but is afraid to say it. She might be trying to get you to make the first move. When you are just hanging around without shirts on does she ever go bra-less? When you are changing together does she usually stand face-on when she is naked? Are you good enough friends to be able to ask her to undress before you talk? And would she hug you naked? Lastly, is it worth the risk to you? (How badly do you need to know?)
Your story raises a lot of questions: Why do you seem to be hugging so often? Where are you when you are together partially undressed? Why are you changing clothes together? Why do you do it so that you can see each other naked? Does Mia's body turn you on? Do you turn her on? Did you ever hug while changing? Do you want to have sex with her or she with you?
Without knowing the answer to these questions it is hard to know the complete answer. Certainly there is a problem here or you would not be wondering if it were healthy. Normally for two 14 year olds to be so casual about their bodies suggests that you are closer than you realize. That kind of behavior requires a great deal of trust; the kind that comes from a deep friendship or a love relationship.
Are you in love? Or are you both enjoying the game of each other's bodies suppressing the stronger sexual feelings so that you can stay friends? If Mia was serious about you staring at her boobs, then she probably enjoys the game but is afraid of getting more physically and/or emotionally involved. If she was not, then she was probably happy that you were looking at her body. So what do you do?
Talk about it, honestly. In your case I might suggest something that I would not normally dare to suggest to two 14 year olds.
If you ask her how she feels and she says that she is not sure, and she asks you and you are not sure then make use of your special relationship - get undressed. You are lucky, because you can do this. Take some time just to look at each other's body. Stare at her and let her stare at you. Put your clothes aside and just look - maybe hug or touch each other briefly - take a little time. Then sit down and try talking again without any clothes on. If you are honest with each other you will be able to admit attraction, emotion and fear. Don't make any decisions while you are naked. DON'T HAVE SEX!!!! End with a long hug. Agree that even if you feel yourself getting turned on don't stop for a few minutes. Then get dressed and be apart for a while.
If you discover new things about yourself take time to think about them before you make any decisions. Talk some more later with your clothes on (after you have been apart for a few hours or a day or two) .
The reason I would dare to suggest this is because the two of you are engagaed in behavior that usually does not occur except between bf/gf or friends with benefits, unlesss you are both nudists. By pushing this behavior to the limit you will know how you feel emotionally and sexually. What is most important is that you are honest with yourself and with each other. If you go through all of this and you discover that you really don't want to be lovers, then you are probably better friends than you thought and mutual nudity is just a sign of that. At least then you will know for sure, and the ability to be naked together or to look at each other will just be something special to you two.
One last important thing. If you decide that you want to start having sex either as bf/gf or just as good friends wait for at least a few weeks during which there is no hugging and all clothes stay on all the time. Then, if you still want to try it, start slow; hand jobs, blow jobs, just fooling around (sexual flirting). Give it time, once you do have sex there is no going back. (AND USE PROTECTION!)