If she's on the waiting list for an autism assessment then I think you will get a better understanding once that process has complete, if you've not much experience with autism.
There are lots of reasons someone with autism might be a picky eater.
There is a genuine NEED, not want or preference, for familiarity, routine and structure. Without it, mental health outcomes become extremely poor, and mental health outcomes are the worst statistically for intellectual autistic girls and women. At 12, one of the only things you can control is what you eat. You don't get much control over anything else.
Because of this, I would let her eat what she feels happiest eating, because the alternative is a heavily restricted diet to the point of anorexia, as it becomes an intense hyperfocus, especially if you make it about weight.
You can't tackle poor eating habits the same way as you would a NT person, because you can't feed or starve rigid thinking out of somebody.
Another reason is sensory. In our area, sensory profiling is usually done post-diagnosis, but you can do private sensory profiles, and it can help you better understand what sorts of foods and textures and flavours to try, in order to maximise nutritional value per meal.
I can also tell you now that the ARFID clinic primary approach is that a fed child is a happy child. When my brother was under the ED services for ARFID some 15-18 years ago, the advice they gave my mum was if he will only eat bacon and chocolate, that's what you give him. I thought surely the advice will have changed, but now my own son has been under them, they've said pretty much the same thing, if there are things he will eat, then that's what you give him. Even if some days that is just ice lollies, and other days it's super noodles, and then other days it's extortionate mcdonalds chicken nuggets.
Another reason the sensory profiling would benefit is because one of our senses is called interoception and that's the sense to tell what's going on inside our body, whether that's pain, hunger, thirst etc. and sometimes when you're autistic this sense can by hyper or hyposensitive, meaning you're either never hungry and can forget to eat, or you can be so hungry that you could eat a 3 course meal and still feel like you're starving because your body can't recognise the internal hunger and fullness signals properly. Placing too much emphasis on her weight won't necessarily make this sense regulate. It will make her feel flawed and ruin her self esteem.
You can ask to speak to a dietician, they can sometimes offer some reassurance and talk to you about different feeding methods.
The method we were told to go with was put everything on one tray that we wanted him to eat alongside the things he had chosen. So that might look like a bowl of noodles, an apple, some chopped strawberries, a bun, and a multivitamin fizzy drink, and just set it down, don't talk about it, let 'em eat what they want from it, and take it away after a few hours. Repeat for each meal. At least that way you're minimising pressure, and maximising the likelihood that something from all food groups will be consumed.