She looks to be opaque, hard plastic, which means that you could probably sunfade her :) For her dull hair, you could try scrubbing it with a toothbrush and dish soap. That gets dirt off. You could condition it too. Good luck, she looks like she might need a lot of work!
Starshine has two different versions. One has more translucent, squishy plastic that usually has a sheen and one is opaque and harder, usually more matte. The harder ones usually stay white and don't sunburn while the translucent ones burn, and are usually brown by now. The opaque ones are usually slightly bigger too. If you search her on google you can see some differences. Yours looks to be white and opaque so she probably won't burn :) If she is squishy and translucent irl, you probably shouldn't though.
I'm just wondering did you take her head off to clean her? Is she sticky inside?
She's a difficult case because her plastic is breaking down hence the brown spots. If plastic breakdown is already underway I'd avoid sunfading in case it speeds up the process. I don't tend to sunfade any white ponies in case they burn but it sounds as though Safflower might know more.
I would suggest removing the head with hot water then a basic clean inside and out, then try scrubbing at the brown water marks with toothpaste or a mild abrasive (never bleach) dry thoroughly and put back together. This might stabilise the plastic and slow her decay. It won't reduce the appearance of the brown spots unfortunately but it might reduce the appearance of the other brown marks.
Symbol rust is impossible to fix as far as I know, unfortunately. The eye rub is easily fixable.
If she wasn't a pegasus I'd suggest findng some clothes for her that might make her flaws less conspicuous.
It's a shame, I've seen so many Starshines like this with great hair but flawed plastic.
Let us know how you get on.