Why is my chicken balding by her vent?

[SwampPrincessChick]

It's almost as though the saddle may have made it worse.

An apron can make the hen's feathers worse because it rubs on her back . . . not as bad as a rooster scratching her back up but it does irritate the skin.

Should I be treating her with anything?

I am not sure how to answer this. That would be a question for someone more experienced on treating health issues, I do not study what to use on chickens because I always forget what the medication or whatever is called.

I'm very concerned as I have a "teenager" rooster, that is starting to try mounting some of the hens, and he's pretty rough.

Your best option is to separate the cockerel from the hens and do not put any aprons on the hens to let their feathers and skin heal. He will only make them stressed out when they already have bare backs. I have seen too many females have bloodied backs from overbreeding males, and that is a major stressor.

My hen to roo ratio is a little low. I was at 5 to 1, I now have 7 to 1 (two pullets the same age as the rooster were hatched at the same time)

That is definitely too low. A low hormone rooster can take five hens, and if he is really nice to them, even lower than that. A normal rooster needs about ten hens. A high hormone rooster needs over ten hens, preferably fifteen. But a rooster will always have his favorite, unfortunately. And the favorite is almost always going to be the lowest-ranking, weakest hen he can find, because that will be the most submissive hen.