Hello and thanks for trusting me to help you and your pet today. I'm a veterinarian with over 25 years experience and would be happy to work with you.
What it sounds as if you are describing is a mosquito bite hypersensitivity. These commonly occur on the ears of cats and is more severe than a typical mosquito bite would be to us as the cat is actually allergic to the bite. Then, when they scratch at it, it only aggravates the situation and can even lead to infection.
The best way to treat this is by keeping her from getting the bites in the first place. Being an indoor/outdoor cat, this will be difficult, but it would help to keep her in during the dawn and dusk, the heavier mosquito times. We are limited in what mosquito repellents are safe for use in cats, but here is a link with some useful information.LINK HERE
The only over the counter relief you will be able to provide once she gets the bites is topical hydro cortisone, which is a pretty weak steroid. If you can get her into the vet, they may be able to provide a longer acting steroid injection or prescribe a more effective oral form of the medication.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.